Add extensive tests for the mingw toolchain and remove trailing slash from Arch.

Address Richard Smith comments: remove the trailing seperator from the Arch
variable, implement six mingw_* trees under tools/clangtest/Driver/Inputs
and merge linux and Windows tests into a universal test that uses these trees.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243098 91177308-0d34-0410-b5e6-96231b3b80d8
31 files changed