[Driver] Darwin: Link in the profile runtime archive first

While building a project with code coverage enabled, we can link in
dependencies which export a weak definition of __llvm_profile_filename.

After r306710, linking in the profiling runtime could pull in a weak
definition of this symbol from a dependency, instead of from within the
runtime's archive.

This inconsistency causes issues during API verification, and is also a
practical problem (the symbol would go missing were the dependent dylib
to be switched out). Introduce a LinkFirst runtime link option to make
sure we always search the profiling runtime for this symbol first.

rdar://problem/33271080

Differential Revision: https://reviews.llvm.org/D35385

llvm-svn: 313065