Set capacity of the string passed to push_item_name.

Other callsites already do this, but these two were missed. This avoids
some allocations.

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -331,7 +331,7 @@ impl<'ll, 'tcx> DebugInfoMethods<'tcx> for CodegenCx<'ll, 'tcx> {
             llvm::LLVMRustDIBuilderCreateSubroutineType(DIB(self), fn_signature)
         };
 
-        let mut name = String::new();
+        let mut name = String::with_capacity(64);
         type_names::push_item_name(tcx, def_id, false, &mut name);
 
         // Find the enclosing function, in case this is a closure.

compiler/rustc_codegen_llvm/src/debuginfo/namespace.rs

@@ -28,7 +28,7 @@ pub fn item_namespace<'ll>(cx: &CodegenCx<'ll, '_>, def_id: DefId) -> &'ll DISco
         .map(|parent| item_namespace(cx, DefId { krate: def_id.krate, index: parent }));
 
     let namespace_name_string = {
-        let mut output = String::new();
+        let mut output = String::with_capacity(64);
         type_names::push_item_name(cx.tcx, def_id, false, &mut output);
         output
     };