[clang] [clang] Improve `_Alignas` on a `struct` declaration diagnostic (PR #65638)

[Aaron Ballman via cfe-commits]

================
@@ -5339,16 +5339,22 @@ Decl *Sema::ParsedFreeStandingDeclSpec(Scope *S, AccessSpecifier AS,
         TypeSpecType == DeclSpec::TST_interface ||
         TypeSpecType == DeclSpec::TST_union ||
         TypeSpecType == DeclSpec::TST_enum) {
-      for (const ParsedAttr &AL : DS.getAttributes())
-        Diag(AL.getLoc(), AL.isRegularKeywordAttribute()
-                              ? diag::err_declspec_keyword_has_no_effect
-                              : diag::warn_declspec_attribute_ignored)
-            << AL << GetDiagnosticTypeSpecifierID(DS);
-      for (const ParsedAttr &AL : DeclAttrs)
-        Diag(AL.getLoc(), AL.isRegularKeywordAttribute()
-                              ? diag::err_declspec_keyword_has_no_effect
-                              : diag::warn_declspec_attribute_ignored)
+
+      auto EmitAttributeDiagnostic = [this, &DS](const ParsedAttr &AL) {
+        unsigned DiagnosticId;
+        if (AL.isAlignas() && !getLangOpts().CPlusPlus) {
+          DiagnosticId = diag::warn_attribute_ignored;
+        } else if (AL.isRegularKeywordAttribute()) {
+          DiagnosticId = diag::err_declspec_keyword_has_no_effect;
+        } else {
+          DiagnosticId = diag::warn_declspec_attribute_ignored;
+        }
----------------
AaronBallman wrote:

```suggestion
        unsigned DiagnosticId = diag::warn_declspec_attribute_ignored;
        if (AL.isAlignas() && !getLangOpts().CPlusPlus)
          DiagnosticId = diag::warn_attribute_ignored;
        else if (AL.isRegularKeywordAttribute())
          DiagnosticId = diag::err_declspec_keyword_has_no_effect;
```
Changes for our odd choice coding style and to be slightly more succinct.

https://github.com/llvm/llvm-project/pull/65638