[COFF] Clean up debug option handling

/debug and /debug:dwarf are orthogonal. An object file can contain both
CodeView and DWARF debug info, so the combination of /debug:dwarf and
/debug should generate both DWARF and a PDB, rather than /debug:dwarf
always suppressing PDB creation.

/nopdb is now redundant and can be removed. /debug /nopdb was previously
used to support DWARF, but specifying /debug:dwarf is entirely
equivalent to that combination now.

Differential Revision: https://reviews.llvm.org/D41310

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@320896 91177308-0d34-0410-b5e6-96231b3b80d8