Add C11 *_DECIMAL_DIG.

Before C11 there was only the DECIMAL_DIG definition. As of C11, we now
have one definition per floating point type (e.g. DBL_DECIMAL_DIG).
Change the existing code to define the new versions. To remain backward
compatible, define __DECIMAL_DIG__ as __LDBL_DECIMAL_DIG__.

Also update the tests. It seems that some of the existing test vectors
were incorrect. Change all tests for __DECIMAL_DIG__ to expect
__LDBL_DECIMAL_DIG__. Add tests for *_DECIMAL_DIG for FreeBSD/amd64, as
I happen to have such a system laying around. I've validated that the
values are in sync with <float.h>.

llvm-svn: 230207