Add __has_builtin support for builtin function-like type traits.

Summary:
Previously __has_builtin(__builtin_*) would return false for
__builtin_*s that we modeled as keywords rather than as functions
(because they take type arguments). With this patch, all builtins
that are called with function-call-like syntax return true from
__has_builtin (covering __builtin_* and also the __is_* and __has_* type
traits and the handful of similar builtins without such a prefix).

Update the documentation on __has_builtin and on type traits to match.
While doing this I noticed the type trait documentation was out of date
and incomplete; that's fixed here too.

Reviewers: aaron.ballman

Subscribers: jfb, kristina, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66100

llvm-svn: 368785