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

[Erich Keane via cfe-commits]

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@@ -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:
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erichkeane wrote:

This desperately needs examples of 'common' uses.  

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