[clang] [clang][diagnostics] add '-Wundef-true' warning option (PR #128265)

[Richard Smith via cfe-commits]

================
@@ -260,7 +260,8 @@ static bool EvaluateValue(PPValue &Result, Token &PeekTok, DefinedTracker &DT,
           PP.Diag(PeekTok, diag::warn_pp_undef_identifier) << II;
 
           const DiagnosticsEngine &DiagEngine = PP.getDiagnostics();
-          // If 'Wundef' is enabled, do not emit 'undef-prefix' diagnostics.
+          // If 'Wundef' is enabled, do not emit 'undef-prefix' or 'undef-true'
----------------
zygoloid wrote:

You should pick the diagnostic ID (`warn_pp_undef_identifier` or `warn_pp_undef_true_identifier`) first, then change the code above to emit that diagnostic, and change this test to check that diagnostic, rather than conditionally emitting another diagnostic below.

Branching on whether a diagnostic is enabled is discouraged. We do it here because `-Wundef-prefix=` is a weird special case, but `-Wundef-true` isn't, so it should do the more conventional thing of picking the diagnostic kind in advance. (There's a general principle here that we want it to be possible to enable "too many" warnings and filter them after the fact and still get the same output, as much as possible.)

This is observable in that, with `-Wundef` enabled, the diagnostic for use of `#if true` will display the more precise warning flag `-Wundef-true`.

https://github.com/llvm/llvm-project/pull/128265