[COFF] support /ERRORLIMIT option

Summary:
This adds support for reporting multiple errors in a single invocation of lld-link. The limit defaults to 20 and can be changed with the /ERRORLIMIT command line parameter, or set to unlimited by passing a value of 0.

This is a new attempt after r295507, which was reverted because opening files raced with exiting early, causing the test to be flaky. This version avoids the race by exiting before calling enqueuePath.

Reviewers: pcc, ruiu

Reviewed By: ruiu

Subscribers: llvm-commits, dblaikie

Differential Revision: https://reviews.llvm.org/D31688

llvm-svn: 299496