Use shrink_to_fit instead of = {} assignment

I'm no C++ expert, but it looks like `vec = {}` is not guaranteed to
release memory.

src/input-sections.cc

@@ -421,7 +421,8 @@ void MergeableSection<E>::resolve_contents(Context<E> &ctx) {
     fragments.push_back(parent.insert(ctx, get_contents(i), hashes[i], p2align));
 
   // Reclaim memory as we'll never use this vector again
-  hashes = {};
+  hashes.clear();
+  hashes.shrink_to_fit();
 }
 
 using E = MOLD_TARGET;

src/passes.cc

@@ -633,7 +633,8 @@ void create_output_sections(Context<E> &ctx) {
     osec->shdr.sh_flags = osec->sh_flags;
     osec->is_relro = is_relro(*osec);
     osec->members = flatten(osec->members_vec);
-    osec->members_vec = {};
+    osec->members_vec.clear();
+    osec->members_vec.shrink_to_fit();
   }
 
   // Add output sections and mergeable sections to ctx.chunks