[Syntax] Add minimal TableGen for syntax nodes. NFC

So far, only used to generate Kind and implement classof().

My plan is to have this general-purpose Nodes.inc in the style of AST
DeclNodes.inc etc, and additionally a special-purpose backend generating
the actual class definitions. But baby steps...

Differential Revision: https://reviews.llvm.org/D90540

Author: sam-mccall

clang/include/clang/CMakeLists.txt

@@ -5,3 +5,4 @@ add_subdirectory(Parse)
 add_subdirectory(Sema)
 add_subdirectory(Serialization)
 add_subdirectory(StaticAnalyzer/Checkers)
+add_subdirectory(Tooling/Syntax)

clang/include/clang/Tooling/Syntax/CMakeLists.txt

@@ -0,0 +1,4 @@
+clang_tablegen(Nodes.inc -gen-clang-syntax-node-list
+  SOURCE Nodes.td
+  TARGET ClangSyntaxNodeList)
+