__has_attribute and pragmas

[Aaron Ballman]

The __has_attribute implementation does not pay attention to the
syntax supported by attributes -- instead, it looks to see whether an
attribute is generally known with that spelling. Since pragmas can now
be spelled as attributes, this means __has_attribute(loop) returns
true because of the #pragma loop functionality. Same for unroll.

Should __has_attribute ignore attributes spelled with a #pragma spelling?

~Aaron