Add return value support to C++ thunks. (#5976)

Based on #5948. A couple of tricky parts:

* When generating the C++ side of the thunk, we are given a pointer to
the location to emplace the return value. The only mechanism C++
provides to perform this emplacement is using placement `operator new`,
which requires a library function in the `<new>` header. We handle this
by declaring that library function ourselves, and rely on Clang not
actually needing a definition for it (which the standard library owns).

* On the Carbon side of the thunk, we want to form an initializing
expression as the result of the call. We don't have a way of expressing
in SemIR that an initializing expression performs its initialization by
storing through a pointer, so this PR adds a new initializing
instruction, `InPlaceInit`, to model an initialization that's performed
opaquely in-place.

toolchain/check/import_cpp.cpp

@@ -102,6 +102,34 @@ static auto GenerateCppIncludesHeaderCode(
                   << "\"\n";
     }
   }
+
+  // Inject a declaration of placement operator new, because the code we
+  // generate in thunks depends on it for placement new expressions. Clang has
+  // special-case logic for lowering a new-expression using this, so a
+  // definition is not required.
+  // TODO: This is a hack. We should be able to directly generate Clang AST to
+  // construct objects in-place without this.
+  // TODO: Once we can rely on libc++ being available, consider including
+  // `<__new/placement_new_delete.h>` instead.
+  code_stream << R"(# 1 "<carbon-internal>"
+#undef constexpr
+#if __cplusplus > 202302L
+constexpr
+#endif
+#undef void
+#undef operator
+#undef new
+void* operator new(__SIZE_TYPE__, void*)
+#if __cplusplus < 201103L
+#undef throw
+throw()
+#else
+#undef noexcept
+noexcept
+#endif
+;
+)";
+
   return code;
 }