Hide some symbols to avoid a crash on shutdown when using code coverage

Summary:
gcov / gcda-based profiling crashes when shared libraries are unloaded

Patch by Benoit Belley and test by Marco Castelluccio for Firefox

See https://bugs.llvm.org/show_bug.cgi?id=27224 & https://bugzilla.mozilla.org/show_bug.cgi?id=1401230

Reviewers: davidxl, rnk, void

Subscribers: jessicah, marco-c, belleyb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D38124