No longer crashing with an assert when __has_include or __has_include_next is used outside of a preprocessor directive.  This fixes PR14837.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@172639 91177308-0d34-0410-b5e6-96231b3b80d8
Adding verbiage to the Language Extensions document about __has_include and __has_include_next only being allowed within preprocessor directives.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@172643 91177308-0d34-0410-b5e6-96231b3b80d8
Add testcase missed yesterday. Patch from Paul Robinson.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@172648 91177308-0d34-0410-b5e6-96231b3b80d8