Move reserveExtraSpace to public (#7755)

The method it overrides in the base class is public, so it's already
accessible publicly. This is warned against in clang-tidy 24.

common/raw_string_ostream.h

@@ -39,6 +39,10 @@ class RawStringOstream : public llvm::raw_pwrite_stream {
   auto empty() -> bool { return str_.empty(); }
   auto size() -> size_t { return str_.size(); }
 
+  auto reserveExtraSpace(uint64_t extra_size) -> void override {
+    str_.reserve(str_.size() + extra_size);
+  }
+
  private:
   auto current_pos() const -> uint64_t override { return str_.size(); }
 
@@ -51,10 +55,6 @@ class RawStringOstream : public llvm::raw_pwrite_stream {
     str_.append(ptr, size);
   }
 
-  auto reserveExtraSpace(uint64_t extra_size) -> void override {
-    str_.reserve(str_.size() + extra_size);
-  }
-
   // The actual buffer.
   std::string str_;
 };