[unittests] Split DWARF tests out of PDB, fix standalone build

Split the PDB tests into DWARF test and actual PDB tests, the latter
requiring DIA SDK. Use the new LLVMConfig.cmake LLVM_ENABLE_DIA_SDK
symbol to enable the PDB tests rather than relying on
llvm/Config/config.h private include file that is not available when
building standalone.

Differential Revision: https://reviews.llvm.org/D26249

llvm-svn: 290819