Fixup test in path to use C:\ instead of D:\ which may be mapped to a removable.

Our internal build bot hit a failure in llvm/test/tools/llvm-symbolizer/pdb/missing_pdb.test
because the test was checking for an error message that is emitted when a pdb file is
missing. But when the drive is mapped to a removalable drive (such as a DVD drive) in
Windows, you get a different error message which causes the test to fail.

This fixes the test by changing the drive the missing pdb is expected to be on to C:\
instead of D:\ as that is the drive historically used to install Windows and thus
if present should be a hard drive.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D92787