[clang] [C++20][Modules] Implement P1857R3 Modules Dependency Discovery (PR #107168)

[Hubert Tong via cfe-commits]

================
@@ -466,6 +466,9 @@ def err_pp_embed_device_file : Error<
 
 def ext_pp_extra_tokens_at_eol : ExtWarn<
   "extra tokens at end of #%0 directive">, InGroup<ExtraTokens>;
+def ext_pp_extra_tokens_at_module_directive_eol
+    : Warning<"extra tokens at end of '%0' directive">,
+      InGroup<ExtraTokens>;
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hubert-reinterpretcast wrote:

The name should probably not start with `ext_`.

I'm not sure that having this in the same group as `ext_pp_extra_tokens_at_eol` is the best design. It is more "legitimate" to want to suppress this message than the other one.

Suggestion for the wording:
```
"tokens after semicolon in '%0' directive"
```

https://github.com/llvm/llvm-project/pull/107168