[clang-doc] Use LangOpts when printing types (#120308)

The implementation in the clang-doc serializer failed to take in the
LangOpts from the declaration. As a result, we'd do things like print
`_

[clang-doc] Use LangOpts when printing types (#120308)

The implementation in the clang-doc serializer failed to take in the
LangOpts from the declaration. As a result, we'd do things like print
`_Bool` instead of `bool`, even in C++ code.

Fixes #62970

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[clang-doc] Add test for functions with builtin return types (#120318)

This is a precommit test for #120308, since we lack non-template
functions that use builtin types.