[COFF] Fix interaction between /DEBUG and /PDB

When /DEBUG is not specified, /PDB should be ignored.  When
/DEBUG is specified, a PDB should be output regardless of
whether or not /PDB is specified.  /PDB just overrides the
default name.

This patch implements this behavior, and adds some tests, while
also removing a dead option /DEBUGPDB which was unused in any
code.

Differential Revision: https://reviews.llvm.org/D33302

llvm-svn: 303352