[GCOV] Flush counters before to avoid counting the execution before fork twice...

[GCOV] Flush counters before to avoid counting the execution before fork twice and for exec** functions we must flush before the call

Summary:
This is replacement for patch in https://reviews.llvm.org/D49460.
When we fork, the counters are duplicate as they're and so the values are finally wrong when writing gcda for parent and child.
So just before to fork, we flush the counters and so the parent and the child have new counters set to zero.
For exec** functions, we need to flush before the call to have some data.

Reviewers: vsk, davidxl, marco-c

Reviewed By: marco-c

Subscribers: llvm-commits, sylvestre.ledru, marco-c

Differential Revision: https://reviews.llvm.org/D53593

llvm-svn: 346313