COFF: Create a PDB file with the correct file signature.

Before this patch, we created an empty PDB file if /debug option is
specified. For MSVC linker, such PDB file is completely broken, and
linker exits without doing anything as soon as it finds an empty PDB
file.

A PDB file created in this patch has the correct file signature.
MSVC linker still thinks that the file is broken, but it then removes
and replaces with its output.

This is an initial patch to support PDB in LLD. We aim to support
PDB in order to make it 100% compatible with MSVC linker. PDB support
is the last missing piece.

llvm-svn: 254796