Switch -mcount and -finstrument-functions to emit EnterExitInstrumenter attributes

This updates -mcount to use the new attribute names (LLVM r318195), and
switches over -finstrument-functions to also use these attributes rather
than inserting instrumentation in the frontend.

It also adds a new flag, -finstrument-functions-after-inlining, which
makes the cygprofile instrumentation get inserted after inlining rather
than before.

Differential Revision: https://reviews.llvm.org/D39331

llvm-svn: 318199