Automatically identify LLVM gcno/gcda files by version

[marco-c]

@jcranmer

[jcranmer]

LLVM uses 402* as its version string for the GCNO/GCDA version (probably because it was meant to be compatible with then-extant gcc on Apple systems, which would be gcc 4.2). Given that we're not likely to be running code that old on gcc (even when I started 7 years ago, we were on gcc 4.4 on Linux), using 402* versus newer versions seems to make sense.

[marco-c]

This is great, thanks. It will solve the problem of builds that mix LLVM and GCC (that is, mozilla-central with Rust code).