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/map_data.rs

@@ -1,4 +1,4 @@
-pub use super::ffi::*;
+use crate::coverageinfo::ffi::{Counter, CounterExpression, ExprKind};
 
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::bug;