Support for reading program counts in llvm-cov.

llvm-cov will now be able to read program counts from the GCDA file and
output it in the same format as gcov. The program summary tag was
identified from gcov-io.h as "\0\0\0\a3".

There is currently a bug in GCOVProfiling.cpp which does not generate
the
run- or program-counting IR, so this change was tested manually by
modifying the GCDA file and comparing the gcov and llvm-cov outputs.

llvm-svn: 193389