[clang] clang: normalise inputs for Windows more aggresively (PR #181306)

[Yanzuo Liu via cfe-commits]

================
@@ -38,6 +38,28 @@ using namespace clang;
 
 #define DEBUG_TYPE "file-search"
 
+namespace {
+void NormalizeCacheKey(StringRef &Path, std::optional<std::string> &Storage) {
----------------
zwuis wrote:

And use `static` instead of anonymous namespaces.

https://llvm.org/docs/CodingStandards.html#restrict-visibility

https://github.com/llvm/llvm-project/pull/181306