Build each testcase variant in its own subdirectory and remove the srcdir lock file
This patch creates a <test>.dwarf, <test>.dwo, etc., build directory for each testcase variant.
Most importantly, this eliminates the need for the per-test lock file in the source directory.
Tests that are marked as NO_DEBUG_INFO_TESTCASE and build with
buildDefault() are built in a <test>.default build directory.
Differential Revision: https://reviews.llvm.org/D42763