Fix mangling collision for C++ class template specializations (#7269)

Carbon-side thunks (for example the `Copy`/`Destroy` witness thunks
generated for imported C++ types) are mangled by Carbon, and their names
incorporate a fingerprint of the involved types. The instruction
fingerprinter identifies a class only by its name and parent scope,
which is sufficient for Carbon classes but not for imported C++ classes:
different specializations of one class template (and other cases such as
types in anonymous namespaces) share a Carbon name and parent scope. As
a result, the thunks for two distinct specializations could mangle to
the same name, producing a single LLVM function with two definitions and
failing `verifyModule` during lowering.

When fingerprinting a class imported from C++, also include the Clang
mangled name of its type.

Test: toolchain/lower/testdata/interop/cpp/thunks.carbon gains a split
with two specializations of one class template, each requiring a thunk;
their thunks now get distinct mangled names instead of colliding.

Assisted-by: Claude Code

---------

Co-authored-by: Christopher Di Bella <cjdb.ns@gmail.com>

toolchain/check/cpp/generate_ast.cpp

@@ -902,6 +902,10 @@ auto GenerateAst(Context& context,
     return false;
   }
 
+  // The AST context is now available, so the mangle context (used to compute
+  // stable identities for imported C++ types) can be created.
+  context.sem_ir().cpp_file()->CreateMangleContext();
+
   auto& ast = clang_instance.getASTContext();
   llvm::IntrusiveRefCntPtr<clang::ExternalSemaSource> carbon_source =
       llvm::makeIntrusiveRefCnt<CarbonExternalASTSource>(&context);