[Driver] Export symbols needed to use profile runtime

Apple's API verification tool (tapi) checks that the symbols exported
from a program match a whitelist. When the program uses the profile
runtime, some additional symbols which are typically not part of the
regular whitelist must be exported.

If we're using symbol export directives along with the profile runtime
on Darwin, the driver needs to export the additional symbols to avoid
verification failures.

rdar://problem/30067753

llvm-svn: 315518