[clang] [llvm] [clang] Add `-verify-directives` cc1 flag (PR #179835)

[Vlad Serebrennikov via cfe-commits]

================
@@ -3476,6 +3494,42 @@ diagnostic verifier will pass. However, if the expected error does not appear
 or appears in a different location than expected, or if additional diagnostics
 appear, the diagnostic verifier will fail and emit information as to why.
 
+Directive Syntax
+~~~~~~~~~~~~~~~~
+EBNF syntax description of the directives is the following:
----------------
Endilll wrote:

I added several examples.
I have to say I'm not sure this grammar is even needed, but it felt awkward that subsequent text was talking about various features of `-verify` without giving a proper frame of reference.

https://github.com/llvm/llvm-project/pull/179835