Respect llvm.used in Internalize.

The language reference says that:

"If a symbol appears in the @llvm.used list, then the compiler,
assembler, and linker are required to treat the symbol as if there is
a reference to the symbol that it cannot see"

Since even the linker cannot see the reference, we must assume that
the reference can be using the symbol table. For example, a user can add
__attribute__((used)) to a debug helper function like dump and use it from
a debugger.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187103 91177308-0d34-0410-b5e6-96231b3b80d8