llvm: cleanup code no longer needed with opaque pointers

When using llvm opaque pointers, typed pointers and pointer bitcasts are
no longer needed.  This also avoids needing packed struct layouts that
are nested inside pointers, letting us avoid computing struct layouts
in Sema that could cause unnecessary dependency loops.

src/codegen/llvm/bindings.zig

@@ -287,9 +287,6 @@ pub const Type = opaque {
     pub const getUndef = LLVMGetUndef;
     extern fn LLVMGetUndef(Ty: *Type) *Value;
 
-    pub const pointerType = LLVMPointerType;
-    extern fn LLVMPointerType(ElementType: *Type, AddressSpace: c_uint) *Type;
-
     pub const arrayType = LLVMArrayType;
     extern fn LLVMArrayType(ElementType: *Type, ElementCount: c_uint) *Type;