Add coverage mapping generation.

This patch adds the '-fcoverage-mapping' option which
allows clang to generate the coverage mapping information
that can be used to provide code coverage analysis using
the execution counts obtained from the instrumentation 
based profiling (-fprofile-instr-generate).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214752 91177308-0d34-0410-b5e6-96231b3b80d8