[clang-tidy] Various improvements in misc-use-override

  * Better error message when more than one of 'virtual', 'override' and 'final'
    is present ("X is/are redundant since the function is already declared Y").
  * Convert the messages to the style used in Clang diagnostics: lower case
    initial letter, no trailing period.
  * Don't run the check for files compiled in pre-C++11 mode
    (http://llvm.org/PR22638).

llvm-svn: 230765