[clang] Support target names with dots in more utilities (PR #65812)

[Fangrui Song via cfe-commits]

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@@ -390,6 +390,21 @@ StringRef stem(StringRef path, Style style = Style::native);
 /// @result The extension of \a path.
 StringRef extension(StringRef path, Style style = Style::native);
 
+/// Get the program's name
+///
+/// If the path ends with the string .exe, returns the stem of
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MaskRay wrote:

.exe => ".exe"

https://github.com/llvm/llvm-project/pull/65812