[clang] [clang-tools-extra] [clangd][clang-tidy] Factor out mergeFixits() logic and reuse it in clangd (PR #208682)

[Zeyi Xu via cfe-commits]

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@@ -803,17 +803,26 @@ void StoreDiags::HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
       }
       // Otherwise, follow clang's behavior: no fixits in macros.
       if (FixIt.RemoveRange.getBegin().isMacroID() ||
-          FixIt.RemoveRange.getEnd().isMacroID())
+          FixIt.RemoveRange.getEnd().isMacroID() ||
+          !isInsideMainFile(FixIt.RemoveRange.getBegin(), SM))
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zeyi2 wrote:

Nit: Could we keep the main-file check separate from the macro guard? It is unrelated to the preceding comment.

https://github.com/llvm/llvm-project/pull/208682