[clang] [clang] Track function template instantiation from definition (PR #110387)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

I pushed a new implementation for this fix.

I have updated the implementation of InstantiateFunctionDefinition so that it doesn't depend anymore on having the exact function declaration instantiation which has the same template context as the definition.

This avoids the more expensive aspect of the previous approach.
While keeping track of separate function declaration instantiations per primary template redeclaration increases source representation accuracy, It would be better to, in the future, go that way across that board for all templates, not just function templates, so that the impact can be better represented.

https://github.com/llvm/llvm-project/pull/110387