[libc++abi] Allow use just compiled clang++ for tests

Currently, the tests assume the system compiler is the one we want
to test, but if we build libcxxabi together with LLVM+Clang, it'll
get the wrong compiler.

This patch allows us to test if we have clang++ in our /bin directory,
and if so, use it.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@243752 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed