[clang] Type safety tweak for AttributeCommonInfo::Form

This patch adds static functions for constructing most
AttributeCommonInfo::Forms.  Direct construction is only retained where
all fields (currently the syntax and spelling) are specified explicitly.

This is a wash on its own.  The purpose is to allow extra fields
to be added to Form without disrupting all callers.  In particular,
it allows extra information to be stored about keywords without
affecting non-keyword uses.

No functional change intended.

Differential Revision: https://reviews.llvm.org/D148104