Use 'override/final' instead of 'virtual' for overridden methods

Summary:
The patch is generated using clang-tidy misc-use-override check.

This command was used:

  tools/clang/tools/extra/clang-tidy/tool/run-clang-tidy.py \
      -checks='-*,misc-use-override' -header-filter='llvm|clang' -j=32 -fix

  svn diff | clang-format-diff -i

Reviewers: dblaikie

Reviewed By: dblaikie

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8927

llvm-svn: 234681