Introduce explicit add_unittest_with_input_files target for tests that use...

Introduce explicit add_unittest_with_input_files target for tests that use llvm::getInputFileDirectory()

Using llvm::getInputFileDirectory() in unit tests is discouraged, so require an explicit opt-in.
This way, cmake also writes ~60 fewer unused files to disk.

Differential Revision: https://reviews.llvm.org/D52095

llvm-svn: 342248