[gcov] Fix .gcda decoding and support GCC 8, 9 and 10

GCDAProfiling.c unnecessarily writes function names to .gcda files.
GCC 4.2 gcc/libgcov.c (now renamed to libgcc/libgcov*) did not write function
names. gcov-7 (compatible) crashes on .gcda produced by libclang_rt.profile
rL176173 realized the problem and introduced a mode to remove function
names.

llvm-cov code apparently takes GCDAProfiling.c output format as truth
and tries to decode function names.  Additionally, llvm-cov tries to
decode tags in certain order which does not match libgcov emitted .gcda
files.

This patch fixes the .gcda decoder and makes it work with GCC 8 and 9
(10 is compatible with 9). Note, line statistics are broken and not
fixed by this patch.

Add test/tools/llvm-cov/gcov-{4.7,8,9}.c to test compatibility.