Adds a basic MinGW32 driver to Clang with GCC-version-independent installation detection that enables building using MinGW in non-hardcoded paths.
As there were breaking changes in trunk, the diff submitted is for release 3.4 as I am unable to build trunk with or without it (and therefore unable to test it). At the moment the patch will not, by default, search "C:\MinGW" as was the old behaviour. However, if necessary, implementing it should be trivial.
This implementation does inherit from Generic_GCC. It successfully detects GCC version 4.8.1 on Windows 7 x64 when given a sysroot of "C:\MinGW\x86_64-posix-4.8.1-rev5" (I should emphasise that the "4.8.1" is not derived from the the directory name).