Get test-suite configuring on Windows with clang-cl

This patch gets the test suite "working" on Windows, although
none of the tests pass.

In order to reuse the existing configuration, which uses UNIX
style flags not accepted by clang-cl, this patch only works with clang++.
When clang-cl is specified the test harness secretly looks for
clang++ and then it configures it using the INCLUDE and LIB enviroment
variables.

This is very much a work in progress.

llvm-svn: 291072