llvm/test/lit.cfg: Retweak for Win32 to fix testing.

llvm/test/lit.cfg: Retweak for Win32 to fix testing.

  - execute_external should be;
    - Not on Win32.
    - Using bash.
    In reverse, "execute_internal" shoud be (Win32 && !bash).

  - lit.getBashPath() behaves differently before and after tweaking $PATH.

I will add a few explanations there later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159641 91177308-0d34-0410-b5e6-96231b3b80d8