[PECOFF] Rename lld::coff -> lld::pecoff.

We had lld::coff and lld::pecoff namespaces for no reason. Unify them.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197201 91177308-0d34-0410-b5e6-96231b3b80d8