[asan] Fix tests for exported interfaces.

Add "OPTIONAL" comment to declaration of weak function in the internal
interface. This fix the tests `interface_symbols_linux.c` and
`interface_symbols_darwin.c` which were failing after r293423.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@293442 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed