coverage: Consolidate FFI types into one module

Coverage FFI types were historically split across two modules, because some of
them were needed by code in `rustc_codegen_ssa`.

Now that all of the coverage codegen code has been moved into
`rustc_codegen_llvm` (#113355), it's possible to move all of the FFI types into
a single module, making it easier to see all of them at once.

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -1,9 +1,8 @@
 use crate::common::CodegenCx;
 use crate::coverageinfo;
-use crate::coverageinfo::map_data::{Counter, CounterExpression};
+use crate::coverageinfo::ffi::{Counter, CounterExpression, CounterMappingRegion};
 use crate::llvm;
 
-use llvm::coverageinfo::CounterMappingRegion;
 use rustc_codegen_ssa::traits::ConstMethods;
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir::def::DefKind;