[clang-tidy] Add override and remove virtual

See
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-override.html
for more on this warning.

Bug: v8:10488
Change-Id: I6805441fa24c80776cf94d18ff389c4c67bb5abd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2186635
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67640}

src/torque/utils.h

@@ -340,7 +340,7 @@ void EraseIf(Container* container, F f) {
 
 class NullStreambuf : public std::streambuf {
  public:
-  virtual int overflow(int c) {
+  int overflow(int c) override {
     setp(buffer_, buffer_ + sizeof(buffer_));
     return (c == traits_type::eof()) ? '\0' : c;
   }