[llvm] Allow GlobalOp to take a region for complex initializers

This allows GlobalOp to either take a value attribute (for simple constants) or a region that can
contain IR instructions (that must be constant-foldable) to create a ConstantExpr initializer.

Example:
  // A complex initializer is constructed with an initializer region.
  llvm.mlir.global constant @int_gep() : !llvm<"i32*"> {
    %0 = llvm.mlir.addressof @g2 : !llvm<"i32*">
    %1 = llvm.mlir.constant(2 : i32) : !llvm.i32
    %2 = llvm.getelementptr %0[%1] : (!llvm<"i32*">, !llvm.i32) -> !llvm<"i32*">
    llvm.return %2 : !llvm<"i32*">
  }
PiperOrigin-RevId: 278717836