[asan] Prevent __attribute__((annotate)) triggering errors on Darwin

The following code triggers a fatal error in the compiler instrumentation
of ASan on Darwin because we place the attribute into llvm.metadata section,
which does not have the proper MachO section name.

void foo() __attribute__((annotate("custom")));
void foo() {;}

This commit reorders the checks so that we skip everything in llvm.metadata
first. It also removes the hard failure in case the section name does not
parse. That check will be done lower in the compilation pipeline anyway.

(Reviewed in http://reviews.llvm.org/D9093.)

llvm-svn: 239379