coverage: Pre-allocate enough capacity for all of a function's mapping regions

Author: Zalathar

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -166,7 +166,7 @@ fn encode_mappings_for_function<'a>(
     }
 
     let mut virtual_file_mapping = Vec::new();
-    let mut mapping_regions = Vec::new();
+    let mut mapping_regions = Vec::with_capacity(counter_regions.len());
 
     // Convert the list of (Counter, CodeRegion) pairs to an array of `CounterMappingRegion`, sorted
     // by filename and position. Capture any new files to compute the `CounterMappingRegion`s