Add --unwindlib=[libgcc|compiler-rt] to parallel --rtlib= [take 2]

Add --unwindlib=[libgcc|compiler-rt] to parallel --rtlib= [take 2]

"clang++ hello.cc --rtlib=compiler-rt"

now can works without specifying additional unwind or exception
handling libraries.

This reworked version of the feature no longer modifies today's default
unwind library for compiler-rt: which is nothing. Rather, a user
can specify -DCLANG_DEFAULT_UNWINDLIB=libunwind when configuring
the compiler.

This should address the issues from the previous version.

Update tests for new --unwindlib semantics.

Differential Revision: https://reviews.llvm.org/D59109

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@356508 91177308-0d34-0410-b5e6-96231b3b80d8