[MC] Don't error on numberless .file directives on MachO

Summary:
Before r349976, MC ignored such directives when producing an object file
and asserted when re-producing textual assembly output. I turned this
assertion into a hard error in both cases in r349976, but this makes it
unnecessarily difficult to write a single assembly file that supports
both MachO and other object formats that support .file. A user reported
this as PR40578, and we decided to go back to ignoring the directive.

Fixes PR40578

Reviewers: mstorsjo

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57772

llvm-svn: 353218