Add rustc_fluent_macro to decouple fluent from rustc_macros

Fluent, with all the icu4x it brings in, takes quite some time to
compile. `fluent_messages!` is only needed in further downstream rustc
crates, but is blocking more upstream crates like `rustc_index`. By
splitting it out, we allow `rustc_macros` to be compiled earlier, which
speeds up `x check compiler` by about 5 seconds (and even more after the
needless dependency on `serde_json` is removed from
`rustc_data_structures`).

compiler/rustc_parse/src/lib.rs

@@ -20,7 +20,7 @@ use rustc_ast_pretty::pprust;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, Diagnostic, FatalError, Level, PResult};
 use rustc_errors::{DiagnosticMessage, SubdiagnosticMessage};
-use rustc_macros::fluent_messages;
+use rustc_fluent_macro::fluent_messages;
 use rustc_session::parse::ParseSess;
 use rustc_span::{FileName, SourceFile, Span};