[clang] [Clang] Eagerly instantiate used constexpr function upon definition. (PR #73463)

[Richard Smith via cfe-commits]

zygoloid wrote:

I *think* the new map should be unnecessary: `FunctionTemplateDecl` and `VarTemplateDecl` already have a list of specializations that you can walk and check the point of instantiation and template specialization kind to see if there's a pending instantiation.

If you want/need to keep the map approach, you'll need to also deal with the case where the template is declared in a PCH and then defined somewhere that includes the PCH, which will mean serializing and deserializing the map contents.

https://github.com/llvm/llvm-project/pull/73463