[patch] Respect llvm.used in Internalize

[Rafael Espíndola]

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 debug
helper function like dump might be referenced from a debugger.

The attached patch makes internalize respect llvm.used.

Cheers,
Rafael
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