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/compile_error.rs

@@ -1,7 +1,7 @@
 // The compiler code necessary to support the compile_error! extension.
 
+use crate::util::get_single_str_from_tts;
 use rustc_ast::tokenstream::TokenStream;
-use rustc_expand::base::get_single_str_from_tts;
 use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacroExpanderResult};
 use rustc_span::Span;