Driver: introduce GNU spellings to control MSVC paths

Add a set of `-Xmicrosoft` flags to control the Windows SDK and VisualC
tools directories.  This allows control over the selection of the SDK
and tools when using the GNU driver.

Differential Revision: https://reviews.llvm.org/D145007
Reviewed By: mstorjo

Author: compnerd

clang/include/clang/Driver/Options.td

@@ -7044,6 +7044,16 @@ def _SLASH_Gv : CLFlag<"Gv">,
 def _SLASH_Gregcall : CLFlag<"Gregcall">,
   HelpText<"Set __regcall as a default calling convention">;
 
+// GNU Driver aliases
+
+def : Separate<["-"], "Xmicrosoft-visualc-tools-root">, Alias<_SLASH_vctoolsdir>;
+def : Separate<["-"], "Xmicrosoft-visualc-tools-version">,
+    Alias<_SLASH_vctoolsversion>;
+def : Separate<["-"], "Xmicrosoft-windows-sdk-root">,
+    Alias<_SLASH_winsdkdir>;
+def : Separate<["-"], "Xmicrosoft-windows-sdk-version">,
+    Alias<_SLASH_winsdkversion>;
+
 // Ignored:
 
 def _SLASH_analyze_ : CLIgnoredFlag<"analyze-">;