Revert r185872 - "Stop emitting weak symbols into the "coal" sections"

This patch broke `make check-asan` on Mac, causing ld warnings like the following one:

ld: warning: direct access in __GLOBAL__I_a to global weak symbol
___asan_mapping_scale means the weak symbol cannot be overridden at
runtime. This was likely caused by different translation units being
compiled with different visibility settings.

The resulting test binaries crashed with incorrect ASan warnings.

llvm-svn: 185923