[sancov] improved object files handling.

Multi-dso programs result in multiple coverage files dumped of the form
'<module_name>.<pid>.sancov'. When analyzing these coverage files it is
important to use correct corresponding object file.

This change removes the "-obj" sancov flag and lets user specify object
file names alongside coverage files. Sancov tool would match them using
<module_name> part of coverage file and short file name of the object
file.

Corresponding changes:
- compiler-rt: http://reviews.llvm.org/D17171
- docs: http://reviews.llvm.org/D17175

Differential Revision: http://reviews.llvm.org/D17169

llvm-svn: 260628