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

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@311355 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb