cpp: index C++ via tree-sitter-cpp, keep .h on C

[SquisherDerHeld]

Indexes C++ sources with tree-sitter-cpp. Files with cpp, cc, cxx, hpp and similar extensions use the C++ grammar; .h stays on C.

Methods store under one lookup key whether defined in class or out of class (~E and E::~E both store as E::~E), template arguments are dropped from keys (Box<T>::get → Box::get), and macro values are re-parsed with the file's own grammar so calls like ns::helper() inside a #define are recorded.

C++ is not on by default; index it with --extensions cpp,h.

Tests: tests/cpp_indexing.rs.

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[SquisherDerHeld]

@facebook-github-bot recheck

[SquisherDerHeld]

This PR is purely AI, all changes look reasonable but I did not look into it too deeply. It just enabled me to index a cpp project

[rikvanriel]

I'm still in the middle of adding proper object oriented language support to the semcode database, so we can handle things like building a call chain through a code base that has multiple functions of the same name, but in different classes.

I'll take a look at whether your approach on how to add C++ can handle the different corner cases that are needed for full support, or whether we need something else.

If we merely need "more", but we do not need to fundamentally change what you did, I'll try to merge this soon. If it turns out we need a different approach for some corner case reason, I'll get to that over time.