[clang-doc] Add support for explicitly typed enums

Add support for explicitly typed enums:
  enum Foo : unsigned { ... };
to the internal representation and to the YAML output.

Add support for getting the value of an enum constant, as well as accessing the original expression that produced it. This changes the YAML output of enums from an array of strings for the enum members to an array of dictionaries. These dictionaries now report the name, value, and original expression.

The markdown and HTML outputs are unchanged, they still output the name from the new enhanced internal schema.

Reviewed By: paulkirth

Differential Revision: https://reviews.llvm.org/D134055