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

[Fangrui Song via llvm-commits]

================
@@ -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
----------------
MaskRay wrote:

.exe => ".exe"

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