Resubmit unittests for DebugInfoPDB.

Resubmit unittests for DebugInfoPDB.

These were originally submitted as part of r228428, but this part
caused a build breakage in LLVMConfig.  The library portion was
resubmitted independently since it was not causing breakage.

There were two reasons this was causing the build to fail.  The
first is that there were no Makefiles added for the PDB tests.  And
the second is that the DebugInfoPDB library was only being built by
CMake behind an "if (MSVC)" check.  This is wrong since this the
library hides platform specific details, and it was causing
LLVM-Config to not find the library when trying to build unittests.

llvm-svn: 228482