[TSan] Add ValgrindSlowdown to the list of exported symbols

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@191952 91177308-0d34-0410-b5e6-96231b3b80d8
[TSan] Automatically pick up llvm-symbolizer from PATH, as we do in another sanitizers

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@191954 91177308-0d34-0410-b5e6-96231b3b80d8