[clang] Fix FIXME in isAlignasAttribute()

AttributeCommonInfo::isAlignasAttribute() was used in one place:
isCXX11Attribute().  The intention was for isAlignasAttribute()
to return true for the C++ alignas keyword.  However, as a FIXME
noted, the function also returned true for the C _Alignas keyword.
This meant that isCXX11Attribute() returned true for _Alignas as
well as for alignas.

AttributeCommonInfos are now always constructed with an
AttributeCommonInfo::Form.  We can use that Form to convey whether
a keyword is alignas or not.

The patch uses 1 bit of an 8-bit hole in the current layout
of AttributeCommonInfo.  This might not be the best long-term design,
but it should be easy to adapt the layout if necessary (that is,
if other uses are found for the spare bits).

I don't know of a way of testing this (other than grep -c FIXME)

Differential Revision: https://reviews.llvm.org/D148105