Document --build-id=fast in ld.lld.1

Initial ld.lld.1 man page commit in r324512 crossed paths with r324502
which added --build-id=fast, allowing --build-id to be documented as a
synonym for --build-id=fast. Catch up with that change in the man page.

Differential Revision:	https://reviews.llvm.org/D43039

llvm-svn: 324528