Driver: Honor %INCLUDE% when built with MinGW

Users are expected to pass system includes through the INCLUDE
environment variable on Windows.  There's no reason to change behavior
based on the toolchain used to build Clang.

I didn't change the registry searching code because I'm not sure it
builds with mingw and I'm not set up to test it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206934 91177308-0d34-0410-b5e6-96231b3b80d8