[InstallAPI] Collect C++ Decls (#84403)

This includes capturing symbols for global variables, functions,
classes, and templated defintions. As pre-determing what symbols are
generated from C++ declarations can be non-trivial, InstallAPI only
parses select declarations for symbol generation when parsing c++.

For example, installapi only looks at explicit template instantiations
or full template specializations, instead of general function or class
templates, for symbol emittion.