[clang-format] Use an enum for context types. NFC

We currently have all those fields in AnnotatingParser::Context.  They
are not inherited from the Context object for the parent scope.  They
are exclusive.  Now they are replaced with an enum.

`InCpp11AttributeSpecifier` and `InCSharpAttributeSpecifier` are not
handled like the rest in ContextType because they are not exclusive.

Reviewed By: curdeius, MyDeveloperDay, HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D121907