LLVM dialect and translation: support global strings

Unlike regular constant values, strings must be placed in some memory and
referred to through a pointer to that memory.  Until now, they were not
supported in function-local constant declarations with `llvm.constant`.
Introduce support for global strings using `llvm.global`, which would translate
them into global arrays in LLVM IR and thus make sure they have some memory
allocated for storage.

PiperOrigin-RevId: 262569316