[GCC] Support for __final specifier

As reported in bug 28473, GCC supports "final" functionality in pre-C++11 code using the __final keyword. Clang currently supports the "final" keyword in accordance with the C++11 specification, however it ALSO supports it in pre-C++11 mode, with a warning.

This patch adds the "__final" keyword for compatibility with GCC in GCC Keywords mode (so it is enabled with existing flags), and issues a warning on its usage (suggesting switching to the C++11 keyword). This patch also adds a regression test for the functionality described. I believe this patch has minimal impact, as it simply adds a new keyword for existing behavior.

This has been validated with check-clang to avoid regressions. Patch is created in reference to revisions 276665.

Patch by Erich Keane.

Differential Revision: https://reviews.llvm.org/D22919

llvm-svn: 277134