Move some functions from rustc_expand to rustc_builtin_macros.

These functions are only used in `rustc_builtin_macros`, so it makes
sense for them to live there. This allows them to be changed from `pub`
to `pub(crate)`.

compiler/rustc_builtin_macros/src/concat_bytes.rs

@@ -1,11 +1,10 @@
+use crate::errors;
+use crate::util::get_exprs_from_tts;
 use rustc_ast::{ptr::P, token, tokenstream::TokenStream, ExprKind, LitIntType, LitKind, UintTy};
-use rustc_expand::base::get_exprs_from_tts;
 use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacEager, MacroExpanderResult};
 use rustc_session::errors::report_lit_error;
 use rustc_span::{ErrorGuaranteed, Span};
 
-use crate::errors;
-
 /// Emits errors for literal expressions that are invalid inside and outside of an array.
 fn invalid_type_err(
     cx: &ExtCtxt<'_>,