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
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.