[PATCH] Mingw-w64 driver for clang

[Alexey Pavlov]

>>! In D5268#44, @rubenvb wrote:
> I have tested basic functionality on Windows using the MinGW-Builds toolchains. Couldn't cleanly test the MSYS2 built ones, but I'm assuming they will function as well. Will try to test on Arch Linux tomorrow.

What problem with testing MSYS2 clang packages?

================
Comment at: lib/Driver/ToolChains.h:764
@@ +763,3 @@
+  bool isPICDefaultForced() const override;
+  bool UseSEHExceptions() const override;
+
----------------
rubenvb wrote:
> This "override" needs to be removed. GCC complains about it, and it is right to do so.
I'm do it here as LLVM guys do for Generic_GCC.

http://reviews.llvm.org/D5268