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`).

src/lib.rs

@@ -27,6 +27,7 @@ extern crate rustc_attr;
 extern crate rustc_codegen_ssa;
 extern crate rustc_data_structures;
 extern crate rustc_errors;
+extern crate rustc_fluent_macro;
 extern crate rustc_hir;
 extern crate rustc_macros;
 extern crate rustc_metadata;
@@ -76,7 +77,7 @@ use rustc_codegen_ssa::target_features::supported_target_features;
 use rustc_codegen_ssa::traits::{CodegenBackend, ExtraBackendMethods, ModuleBufferMethods, ThinBufferMethods, WriteBackendMethods};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::{DiagnosticMessage, ErrorGuaranteed, Handler, SubdiagnosticMessage};
-use rustc_macros::fluent_messages;
+use rustc_fluent_macro::fluent_messages;
 use rustc_metadata::EncodedMetadata;
 use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
 use rustc_middle::ty::TyCtxt;