Driver: introduce GNU spellings to control MSVC paths

[compnerd]

Summary

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.

[mstorsjo]

Looks reasonable to me, adding a few others for opinions. Should we have some tests for this? I guess testing these is somewhat environment reliant though, so it's probably not so easy...

[compnerd]

Yeah, I suspect that would be difficult. Additionally, these are aliases, so they should already have testing coverage through the existing tests.

[compnerd]

Just a friendly reminder, I'd like to get this merged to do a follow up change to improve some of the usability of these flags.

[mstorsjo]

If others don't have anything to add on this matter, I guess this is fine.

The new option names feel rather wordy and unwieldy (especially compared to the clang-cl forms they're aliases for), so if you would want to consider a slightly shorter form, that would also be fine with me. But I won't object to this form either.

clang/include/clang/Driver/Options.td

def _SLASH_Gr : CLFlag<"Gr">,
   HelpText<"Set __fastcall as a default calling convention">;
 def _SLASH_Gz : CLFlag<"Gz">,
   HelpText<"Set __stdcall as a default calling convention">;
 def _SLASH_Gv : CLFlag<"Gv">,
   HelpText<"Set __vectorcall as a default calling convention">;
 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-">;
 def _SLASH_bigobj : CLIgnoredFlag<"bigobj">;
 def _SLASH_cgthreads : CLIgnoredJoined<"cgthreads">;
 def _SLASH_d2FastFail : CLIgnoredFlag<"d2FastFail">;
 def _SLASH_d2Zi_PLUS : CLIgnoredFlag<"d2Zi+">;
 def _SLASH_errorReport : CLIgnoredJoined<"errorReport">;