PECOFF: Move a call of WinLinkDriver::parse from FileCOFF::doParse to FileCOFF::beforeLink

In doParse, we shouldn't do anything that has side effects. That function may be
called speculatively and possibly in parallel.

We called WinLinkDriver::parse from doParse to parse a command line in a .drectve
section. The parse function updates a linking context object, so it has many side
effects. It was not safe to call that function from doParse. beforeLink is a
function for a File object to do something that has side effects. Moving a call
of WinLinkDriver::parse to there.

llvm-svn: 230791