[lldb][dotest] Improve libc++ detection

Summary: The test logic for running libc++ tests only looks to see if `/usr/include/c++/v1` exists. This adds a fallback for including libc++ tests as long as `$(CC) -stdlib=libc++` works.

Reviewers: labath, EricWF

Subscribers: ldionne, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71319