Coverage: add HasCodeBefore flag to a mapping region.

This flag will be used by the coverage tool to help 
compute the execution counts for each line in a source file.

Differential Revision: http://reviews.llvm.org/D4746


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214740 91177308-0d34-0410-b5e6-96231b3b80d8