[gcov] Emit GCOV_TAG_OBJECT_SUMMARY/GCOV_TAG_PROGRAM_SUMMARY correctly and fix llvm...

[gcov] Emit GCOV_TAG_OBJECT_SUMMARY/GCOV_TAG_PROGRAM_SUMMARY correctly and fix llvm-cov's decoding of runcount

gcov 9 (r264462) started to use GCOV_TAG_OBJECT_SUMMARY. Before,
GCOV_TAG_PROGRAM_SUMMARY was used.
libclang_rt.profile should emit just one tag according to the version.

Another bug introduced by rL194499 is that the wrong runcount field was
selected.

Fix the two bugs so that gcov can correctly decode "Runs:" from
libclang_rt.profile produced .gcda files, and llvm-cov gcov can
correctly decode "Runs:" from libgcov produced .gcda files.