[clang-doc] Clean up *Info constructors.

[clang-doc] Clean up *Info constructors.

The *Info object (for the copy of the AST") constructors had many duplicated variants. Many of the variants seemed to be in an attempt to avoid default arguments. But default arguments are not prohibited and using them allows most of the variants to be removed which improves readability.

Remove the IsInGlobalNamespace flag on a Reference. This is set when the path is empty, and only read once in the HTML generator with the identical condition. The constructor cleanup exposed a problem where this was set to false when the constructor with no path was used, but true when the path was set to empty.

There should be no observable change with the exception that IsInGlobalNamespace is no longer emitted in YAML.

Reviewed By: paulkirth, haowei

Differential Revision: https://reviews.llvm.org/D134235