[gn build] Add build file for DebugInfoPDBTests

[gn build] Add build file for DebugInfoPDBTests

I'm pretty unhappy this patch: DebugInfoPDBTests uses an API that requires some
magic txt file to be next to the unit test executable that stores the absolute
path to the LLVM source root.

The choices here are:

1. Don't use the unittest() template for DebugInfoPDBTests and set output_dir
   for unit tests in two places (the gni file for every test but this one, and the
   BUILD.gn file for this specific test).

2. Add another unittest_foo() template variation for this one test.

I went with the former, and added a comment to the template to look out for
this.

(The CMake build has the same issue.)

Differential Revision: https://reviews.llvm.org/D56324

llvm-svn: 350447