Add a text mode for fingerprinting. (#7231)

The intent is to add visibility into how the fingerprint is computed, so
that fingerprinting issues and mangling collisions can be more readily
understood and fixed.

Assisted-by: Gemini via Antigravity

toolchain/check/cpp/export.cpp

@@ -480,7 +480,8 @@ static auto BuildCppFunctionDeclForCarbonFn(Context& context,
   function_decl->setParams(param_var_decls);
 
   // Mangle the function name and attach it to the `FunctionDecl`.
-  SemIR::Mangler m(context.sem_ir(), context.total_ir_count());
+  SemIR::Mangler m(context.sem_ir(), context.total_ir_count(),
+                   context.mangle_string_fingerprint());
   std::string mangled_name = m.Mangle(function_id, SemIR::SpecificId::None);
   function_decl->addAttr(
       clang::AsmLabelAttr::Create(context.ast_context(), mangled_name));