NFC normalize lifetime identifiers

[gmorenz]

Fixes #126759

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[jieyouxu]

This is a observable language change in that previously we didn't accept

struct Foo<'a>(&'a ());
fn foo<'K>(_x: Foo<'K>, _y: Foo<'K>) {}
//      ^- latin K               ^- kelvin K

error[E0261]: use of undeclared lifetime name `'K`
 --> src/lib.rs:3:33
  |
3 | fn foo<'K>(_x: Foo<'K>, _y: Foo<'K>) {}
  |                                 ^^ undeclared lifetime
  |
help: consider introducing lifetime `'K` here
  |
3 | fn foo<'K, 'K>(_x: Foo<'K>, _y: Foo<'K>) {}
  |        +++

So when we do NFC-normalize the lifetimes, we are now accepting more code (but is consistent with the original RFC and regular identifiers). This needs a lang FCP, and probably an update to the reference on NFC-normalization.

@rustbot label: +I-lang-nominated

[gmorenz]

I think the reference is actually fine - I'm not sure if something changed since the original bug report or the original bug report was just a generous reading of the current reference as accepting the non-normalizing implementation.

The lifetime syntax page is here, and simply references the lifetime identifiers as either IDENTIFIER_OR_KEYWORD or NON_KEYWORD_IDENTIFIER as defined here which specifies that they should be normalized.