Missing typo correction diagnostic in preprocessor conditionals

[AaronBallman]

Bugzilla Link	52256
Version	trunk
OS	Windows NT
CC	@zygoloid

Extended Description

Consider the following code, and assume that FOO is not defined:

#ifdef FOO
#elfidef BAR
#endif

In this case, we do not issue any diagnostic about the typo where #elifdef is misspelled as #elfidef because it appears within a preprocessor branch that is discarded.

When scanning for the end of the discarded block, we should consider typo corrections for non-directives to help catch this sort of mistake.

[zygoloid]

We should be a little careful here. It's not unreasonable for people to write

#if special_compiler
#special_compiler_directive
#endif

I think it would be reasonable to warn by default on typos where the correction is to #if/#ifdef/#ifndef/#elif/#elifdef/#elifndef/#else/#endif, but not corrections to any other directive. (We can't produce an error, because such inputs are valid.)

[AaronBallman]

Agreed that we need to be careful. FWIW, what raised the issue for me was when I tested the behavior of #elifdef/#elifndef in older Clang versions and I was... a bit surprised: https://godbolt.org/z/8Wdx5d9of