[asan] Do all calls to __asan_report_* through PLT.

This matters when runtime is built as a shared library. Even though calling
code is itself part of the same library, these symbols are public and can
(theoretically) be interposed.

It might be better to declare hidden aliases for asan_report_* and call them
directly, but
  (a) they are (noreturn), so performance does not matter, and
  (b) it may be potentially less portable.

llvm-svn: 201764