Add new debug kind LocTrackingOnly.

Summary:
This new debug emission kind supports emitting line location
information in all instructions, but stops code generation
from emitting debug info to the final output.

This mode is useful when the backend wants to track source
locations during code generation, but it does not want to
produce debug info. This is currently used by optimization
remarks (-Rpass, -Rpass-missed and -Rpass-analysis).

When one of the -Rpass flags is used, the front end will enable
location tracking, only if no other debug option is enabled.

To prevent debug information from being generated, a new debug
info kind LocTrackingOnly causes DIBuilder::createCompileUnit() to
not emit the llvm.dbg.cu annotation. This blocks final code generation
from generating debug info in the back end.

Depends on D4234.

Reviewers: echristo, dblaikie

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4235

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211610 91177308-0d34-0410-b5e6-96231b3b80d8