[sanitizer] Remove -fno-function-sections from SANITIZER_COMMON_CFLAGS

Summary:
-fno-function-sections was added as a default Sanitizer common cflag with
https://reviews.llvm.org/rL200683, the reasoning behind was that things would
break if linked with --gc-sections.

This appears to not be necessary anymore, as tests pass without, including
function-sections-are-bad.cc. There is a large benefit to having
function-sections when dealing with static libraries in terms of size and
dependencies that go away with --gc-sections.

Reviewers: kcc, eugenis

Reviewed By: eugenis

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D29132

llvm-svn: 293220