[PATCH] clang-cl: Implement the -arch flag

[Hans Wennborg]

lgtm

================
Comment at: lib/Driver/Tools.cpp:1454
@@ +1453,3 @@
+    std::string ArchLower(Arch);
+    std::transform(ArchLower.begin(), ArchLower.end(), ArchLower.begin(), std::tolower);
+    bool ArchUsed = false;
----------------
You can just do:

std::string ArchLower = Arch->lower();

Or maybe just use Arch->lower() when updating Features in the code below.

http://reviews.llvm.org/D4519