[PATCH C++] - SD-6 Implementation Part 1 - __has_include.

[Ed Smith-Rowland <3dw4rd at verizon dot net>]

Greetings,

I am finally getting back to my SD-6 C++ features test work.
This first part adds a __has_include__ built-in that will return true if 
a header exists.
I also added __has_include_next__ as an extension.  Clang has this 
extension.

Both these built-ins will be wrapped in function type macros in a later 
patch to c-family.

As written, these are available to the whole C-family rather than just C++.
I think this makes a valuable addition for everyone.
(I sort of wonder why this wasn't added to the actual preprocessor 20 
years ago.)

Bootstrapped and tested under x86_64-linux.

OK?

Ed

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