coverage: Fetch expressions and mappings separately

The combined `get_expressions_and_counter_regions` method was an artifact of
having to prepare the expressions and mappings at the same time, to avoid
ownership/lifetime problems with temporary data used by both.

Now that we have an explicit transition from `FunctionCoverageCollector` to the
final `FunctionCoverage`, we can prepare any shared data during that step and
store it in the final struct.

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -185,13 +185,13 @@ fn encode_mappings_for_function(
     global_file_table: &mut GlobalFileTable,
     function_coverage: &FunctionCoverage<'_>,
 ) -> Vec<u8> {
-    let (expressions, counter_regions) = function_coverage.get_expressions_and_counter_regions();
-
-    let mut counter_regions = counter_regions.collect::<Vec<_>>();
+    let mut counter_regions = function_coverage.counter_regions().collect::<Vec<_>>();
     if counter_regions.is_empty() {
         return Vec::new();
     }
 
+    let expressions = function_coverage.counter_expressions().collect::<Vec<_>>();
+
     let mut virtual_file_mapping = IndexVec::<u32, u32>::new();
     let mut mapping_regions = Vec::with_capacity(counter_regions.len());