Initial support for C++ function pointers (#7788)

This change introduces a new inst kind `CppFunctionPointerType`, which
represents an imported C++ function pointer that can be invoked from
Carbon (support for forming such a pointer from a Carbon function is in
a follow-up PR). This is implemented by treating the operation of
invoking a function pointer in C++ as if it were a call to an `__invoke`
method on the function pointer type, and extending the existing
function-import logic to support importing this fictitious method.

---------

Co-authored-by: Nicholas Bishop <nbishop@nbishop.net>

toolchain/check/type.cpp

@@ -178,6 +178,12 @@ auto GetClassType(Context& context, SemIR::ClassId class_id,
   return GetTypeImpl<SemIR::ClassType>(context, class_id, specific_id);
 }
 
+auto GetCppFunctionPointerType(Context& context,
+                               SemIR::ClangFunctionPointerTypeId clang_type_id)
+    -> SemIR::TypeId {
+  return GetTypeImpl<SemIR::CppFunctionPointerType>(context, clang_type_id);
+}
+
 auto GetCppOverloadSetType(Context& context,
                            SemIR::CppOverloadSetId overload_set_id,
                            SemIR::SpecificId specific_id) -> SemIR::TypeId {