[unittests] Build LLVMTestingSupport for out-of-source builds

The Process/gdb-remote test now requires the LLVMTestingSupport library
that is not installed by LLVM. As a result, when doing an out-of-source
build it fails being unable to find the library. To solve that, build
a local copy of the library when building LLDB with unittests and LLVM
sources available. This is based on how we deal with bundled gtest
sources.

Differential Revision: https://reviews.llvm.org/D36886

llvm-svn: 311355