[clang] [clang] Do not pass -canonical-system-headers on Windows by default (PR #71097)

[Fangrui Song via cfe-commits]

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@@ -1180,8 +1180,19 @@ void Clang::AddPreprocessingOptions(Compilation &C, const JobAction &JA,
     if (ArgM->getOption().matches(options::OPT_M) ||
         ArgM->getOption().matches(options::OPT_MD))
       CmdArgs.push_back("-sys-header-deps");
+
+      // #70011: Canonicalization on Windows does unexpected things like change
+      // drive letters.
+      // FIXME: find and use Windows API that canonicalizes paths except for
+      // drive letter.
+#if defined(_WIN32) || defined(_WIN64)
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MaskRay wrote:

`_WIN32` is defined on 64-bit Windows.

https://github.com/llvm/llvm-project/pull/71097