[CodeCompletion] Code complete the missing C++11 keywords

This commit adds context sensitive code completion support for the C++11
keywords that currently don't have completion results.

The following keywords are supported by this patch:

alignas
constexpr
static_assert
noexcept (as a function/method qualifier)
thread_local

The following special identifiers are also supported:

final (as a method qualifier or class qualifier)
override

rdar://29219185

Differential Revision: https://reviews.llvm.org/D28286


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@295001 91177308-0d34-0410-b5e6-96231b3b80d8