Flush gcda files before unlocking them

[marco-c]

Summary

Fixes https://bugs.llvm.org/show_bug.cgi?id=35464.

[sylvestre.ledru]

I added two more reviewers as we would like to get that in trunk before 6.0 branches.

FYI, this was necessary to fix some intermittent issues for Firefox code coverage.

[marco-c]

The change is very contained and safe and we need it in 6.0.

lib/profile/GCDAProfiling.c

if (output_file) {
 
     if (new_file) {
       fwrite(write_buffer, cur_pos, 1, output_file);
       free(write_buffer);
     } else {
       unmap_file();
     }
 
+    fflush(output_file);
     flock(fd, LOCK_UN);
     fclose(output_file);
     output_file = NULL;
     write_buffer = NULL;
   }
   free(filename);
 
 #ifdef DEBUG_GCDAPROFILING