Look for both .debug and dsym debugging symbol information for stripped executable.

Currently Symbols::LocateExecutableSymbolFile on MacOSX only looks for external
dsym debugging information, however if running on a stripped dwarf executable it
should also check for a .debug file as well.

Test Plan:
./dotest.py $DOTEST_OPTS -t -p TestSharedLibStrippedSymbols.py
This test now passes when running a remote Mac -> Linux test, and still passes
running locally on Mac or locally on Linux.

Differential Revision: http://reviews.llvm.org/D9174

llvm-svn: 235737