[Clang] Fix ICE where C++ Template Instantiation failed to handle attributed lambdas (#76523)

This PR is proposing a fix for
https://github.com/llvm/llvm-project/issues/76521.

Clang used to assume that during template instantiation, Lambda
expressions can only have `FunctionProtoTypeLoc`s. However, this is not
true for certain attributes like `__attribute__((pcs("aapcs-vfp")))`,
whose interpretation happens after template instantiation. This PR
changes the transformation logic for lambdas.