Use rustc_fluent_macro::fluent_messages! directly.

Currently we always do this:
```
use rustc_fluent_macro::fluent_messages;
...
fluent_messages! { "./example.ftl" }
```
But there is no need, we can just do this everywhere:
```
rustc_fluent_macro::fluent_messages! { "./example.ftl" }
```
which is shorter.

compiler/rustc_codegen_llvm/src/lib.rs

@@ -41,7 +41,6 @@ use rustc_codegen_ssa::ModuleCodegen;
 use rustc_codegen_ssa::{CodegenResults, CompiledModule};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::{ErrorGuaranteed, FatalError, Handler};
-use rustc_fluent_macro::fluent_messages;
 use rustc_metadata::EncodedMetadata;
 use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
 use rustc_middle::ty::TyCtxt;
@@ -92,7 +91,7 @@ mod type_of;
 mod va_arg;
 mod value;
 
-fluent_messages! { "../messages.ftl" }
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 #[derive(Clone)]
 pub struct LlvmCodegenBackend(());