[PATCH] Mingw-w64 driver for clang

[Reid Kleckner]

> I think we need remove subclassing from Generic_GCC and subclass from Toolchain as rubenvb do earlier. And to detect too chains in path write our own functions.

That seems fine.

> Looking into Driver.cpp I see that toolchains trying to find basing on env "COMPILER_PATH" not the "PATH". So seems this code need to change too if we want to find in PATH.

That's a surprise, but we should find a way to factor this out and parameterize it as necessary then. We don't need to inherit from Generic_GCC to reuse the same functionality.

http://reviews.llvm.org/D5268