[compiler-rt] Re-enable the use of -gmlt for ASan tests on Darwin

The optimization for -gmlt/-gline-tables-only introduced in r218129 happened to break on Darwin and produce no line number information due to
an incompatibility with dsymutil. ASan tests have been failing because of that and we disabled the use of -gmlt for the tests in r218545. This patch re-enables the use of -gmlt, because we have conditionally disabled the incompatible optimization in LLVM, so -gmlt now works on Darwin. Once Darwin's dsymutil is modified to allow this optimization, we can re-enable the optimization in LLVM.

llvm-svn: 218716