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/sem_ir/stringify.cpp

@@ -9,6 +9,7 @@
 #include <utility>
 #include <variant>
 
+#include "clang/AST/Type.h"
 #include "common/concepts.h"
 #include "common/raw_string_ostream.h"
 #include "toolchain/base/kind_switch.h"
@@ -654,6 +655,14 @@ class Stringifier {
                       ">");
   }
 
+  auto StringifyInst(InstId /*inst_id*/, CppFunctionPointerType inst) -> void {
+    clang::QualType clang_type(sem_ir_->clang_function_pointer_types()
+                                   .Get(inst.clang_type_id)
+                                   .clang_type,
+                               0);
+    *out_ << "<C++ type " << clang_type.getAsString() << ">";
+  }
+
   auto StringifyInst(InstId /*inst_id*/, FunctionType inst) -> void {
     const auto& fn = sem_ir_->functions().Get(inst.function_id);
     *out_ << "<type of ";