[asancov] Faster coverage in memory-mapped mode.

Use caller pc of __sanitizer_cov_module_init to figure out
when 2 sequential calls are from the same module; skip
.sancov.map file update in this case.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@210267 91177308-0d34-0410-b5e6-96231b3b80d8