[Coverage] Fix an issue where improper coverage mapping data could be loaded...

[Coverage] Fix an issue where improper coverage mapping data could be loaded for an inline function.

If an inline function is observed but unused in a translation unit, dummy
coverage mapping data with zero hash is stored for this function.
If such a coverage mapping section came earlier than real one, the latter
was ignored. As a result, llvm-cov was unable to show coverage information
for those functions.

Differential Revision: http://reviews.llvm.org/D20286

llvm-svn: 270194