[lex.pptoken] Separate out placeholder keywords

The _preprocessing-token_ term is defined as one of another set of
grammar productions, with the three placeholder tokens standing out
as specific single tokens.

Moving the placeholder tokens into their own production simplifies
both the _preprocessing-token_ and _keyword_ grammar productions,
plus it guarantees a consistency for the set of placeholder between
both places.  IT also makes it easier for [cpp] to directly
cross-reference where needed.

Pedantically, the term placeholder token itself is problematic as
tokens are a phase 7 concern, where they become keywords.  Within
the earlier phases where these placeholders are used the correct
term is a preprocessing token, so strictly these should be
placeholder preprocessing tokens, which is a mouthful.  So renaming
the term to placeholder keyword, which is hopefully less of a
concern as there are no keywords in phase 3--6 to confuse with
phase 7 keywords.

Finally, clean up the informative list in text of the various kinds
of preprocessing tokens, rendering as a list in the source (one kind
per line) and adding a cross-reference to each such item.

source/lex.tex

@@ -495,12 +495,10 @@
 \indextext{token!preprocessing|(}%
 \begin{bnf}
 \nontermdef{preprocessing-token}\br
+    placeholder-keyword\br
     header-name\br
-    import-keyword\br
-    module-keyword\br
-    export-keyword\br
-    identifier\br
     pp-number\br
+    identifier\br
     character-literal\br
     user-defined-character-literal\br
     string-literal\br
@@ -515,12 +513,15 @@
 In this document,
 glyphs are used to identify
 elements of the basic character set\iref{lex.charset}.
-The categories of preprocessing token are: header names,
-placeholder tokens produced by preprocessing \tcode{import} and \tcode{module} directives
-(\grammarterm{import-keyword}, \grammarterm{module-keyword}, and \grammarterm{export-keyword}),
-identifiers, preprocessing numbers, character literals (including user-defined character
-literals), string literals (including user-defined string literals), preprocessing
-operators and punctuators, and single non-whitespace characters that do not lexically
+The categories of preprocessing token are:
+placeholder keywords\iref{lex.placeholder},
+header names\iref{lex.header},
+preprocessing numbers\iref{lex.ppnumber},
+identifiers\iref{lex.name},
+character literals\iref{lex.ccon} including user-defined character literals,
+string literals\iref{lex.string} including user-defined string literals,
+preprocessing operators and punctuators\iref{lex.operators}, and
+single non-whitespace characters that do not lexically
 match the other preprocessing token categories.
 If a \unicode{0027}{apostrophe} or a \unicode{0022}{quotation mark} character
 matches the last category, the program is ill-formed.
@@ -548,17 +549,6 @@
 shall have the lexical form of a keyword, an identifier, a literal,
 or an operator or punctuator.
 
-\pnum
-The \grammarterm{import-keyword} is produced
-by processing an \keyword{import} directive\iref{cpp.import},
-the \grammarterm{module-keyword} is produced
-by preprocessing a \keyword{module} directive\iref{cpp.module}, and
-the \grammarterm{export-keyword} is produced
-by preprocessing either of the previous two directives.
-\begin{note}
-None has any observable spelling.
-\end{note}
-
 \pnum
 If the input stream has been parsed into preprocessing tokens up to a
 given character:
@@ -628,6 +618,32 @@
 \end{example}
 \indextext{token!preprocessing|)}
 
+\rSec1[lex.placeholder]{Placeholder keywords}
+\indextext{keyword!placeholder|(}%
+
+\begin{bnf}
+\nontermdef{placeholder-keyword}\br
+    import-keyword\br
+    module-keyword\br
+    export-keyword
+\end{bnf}
+
+\pnum
+Placeholder keywords are produced by preprocessing \keyword{import}
+and \keyword{module} directives.
+The \grammarterm{import-keyword} is produced
+by preprocessing an \keyword{import} directive\iref{cpp.import},
+the \grammarterm{module-keyword} is produced
+by preprocessing a \keyword{module} directive\iref{cpp.module}, and
+the \grammarterm{export-keyword} is produced
+by preprocessing either of the previous two directives.
+
+\pnum
+\begin{note}
+Placeholder keywords do not have any observable spelling.
+\end{note}
+\indextext{keyword!placeholder|)}%
+
 \rSec1[lex.header]{Header names}
 
 \indextext{header!name|(}%
@@ -960,9 +976,7 @@
 \begin{bnf}
 \nontermdef{keyword}\br
     \textnormal{any identifier listed in \tref{lex.key}}\br
-    \grammarterm{import-keyword}\br
-    \grammarterm{module-keyword}\br
-    \grammarterm{export-keyword}
+    \grammarterm{placeholder-keyword}
 \end{bnf}
 
 \pnum

source/preprocessor.tex

@@ -796,7 +796,7 @@
 \pnum
 The \tcode{module} and \tcode{export} (if it exists) preprocessing tokens
 are replaced by the \grammarterm{module-keyword} and
-\grammarterm{export-keyword} preprocessing tokens respectively.
+\grammarterm{export-keyword} placeholder keywords respectively\iref{lex.placeholder}.
 \begin{note}
 This makes the line no longer a directive
 so it is not removed at the end of phase 4.
@@ -854,7 +854,7 @@
 In all three forms of \grammarterm{pp-import},
 the \tcode{import} and \tcode{export} (if it exists) preprocessing tokens
 are replaced by the \grammarterm{import-keyword} and
-\grammarterm{export-keyword} preprocessing tokens respectively.
+\grammarterm{export-keyword} placeholder keywords respectively\iref{lex.placeholder}.
 \begin{note}
 This makes the line no longer a directive
 so it is not removed at the end of phase 4.