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

Description

[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