Introduce a setting to disable Spotlight while running the test suite

This is a more principled approach to disabling Spotlight .dSYM
lookups while running the testsuite, most importantly it also works
for the LIT-based tests, which I overlooked in my initial fix
(renaming the test build dir to lldb-tests.noindex).

Differential Revision: https://reviews.llvm.org/D44342

llvm-svn: 327330