Add unused symbol to thunk files to force wholearchive inclusion

These "dynamic_runtime_thunk" object files exist to create a weak alias
from 'foo' to 'foo_dll' for all weak sanitizer runtime symbols. The weak
aliases are implemented as /alternatename linker options in the
.drective section, so they are not actually in the symbol table. In
order to force the Visual C++ linker to load the object, even with
-wholearchive:, we have to provide at least one external symbol. Once we
do that, it will read the .drective sections and see the weak aliases.

Fixes PR42074

llvm-svn: 362970