[clang] [clang-tools-extra] Reland [clang][Sema, Lex, Parse] Preprocessor embed in C and C++ (PR #95802)

[Aaron Ballman via cfe-commits]

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@@ -436,6 +436,14 @@ def warn_cxx23_compat_warning_directive : Warning<
 def warn_c23_compat_warning_directive : Warning<
   "#warning is incompatible with C standards before C23">,
   InGroup<CPre23Compat>, DefaultIgnore;
+def ext_pp_embed_directive : ExtWarn<
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AaronBallman wrote:

> Is there a reason why this is a ExtWarn and not Extension? 

We're consistently inconsistent; if you look through `DiagnosticParseKinds.td` you'll see we're all over the place. I think the organic rule tends to be whether the patch author thinks the failure mode from not noticing use of the extension will be disruptive or not.

I think for the preprocessor we'd generally use `ExtWarn` for new directives because those are *really* not portable and can lead to some very odd follow-on diagnostics. That's why we went with `ExtWarn` for `#elifdef` (also new in C23), though I think it's less of a concern with `#embed`.

https://github.com/llvm/llvm-project/pull/95802