[CodeGen][WebAssembly] Better lowering for WASM_SYMBOL_TYPE_GLOBAL symbols

As we have been missing support for WebAssembly globals on the IR level,
the lowering of WASM_SYMBOL_TYPE_GLOBAL to IR was incomplete.  This
commit fleshes out the lowering support, lowering references to and
definitions of addrspace(1) values to correctly typed
WASM_SYMBOL_TYPE_GLOBAL symbols.

Depends on D101608.

Differential Revision: https://reviews.llvm.org/D101913