[xray] [tests] Detect and handle missing LLVMTestingSupport gracefully

Add a code to properly test for presence of LLVMTestingSupport library
when performing a stand-alone build, and skip tests requiring it when
it is not present.  Since the library is not installed, llvm-config
reported empty --libs for it and the tests failed to link with undefined
references.  Skipping the two fdr_* test files is better than failing to
build, and should be good enough until we find a better solution.

NB: both installing LLVMTestingSupport and building it automatically
from within compiler-rt sources are non-trivial.  The former due to
dependency on gtest, the latter due to tight integration with LLVM
source tree.

Differential Revision: https://reviews.llvm.org/D55891

llvm-svn: 349899