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/check_unit.cpp

@@ -63,7 +63,7 @@ CheckUnit::CheckUnit(
     llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> fs,
     llvm::LLVMContext* llvm_context,
     std::shared_ptr<clang::CompilerInvocation> clang_invocation,
-    llvm::raw_ostream* vlog_stream)
+    llvm::raw_ostream* vlog_stream, bool mangle_string_fingerprint)
     : unit_and_imports_(unit_and_imports),
       tree_and_subtrees_getter_(tree_and_subtrees_getters->Get(
           unit_and_imports->unit->sem_ir->check_ir_id())),
@@ -75,7 +75,8 @@ CheckUnit::CheckUnit(
       context_(&emitter_, tree_and_subtrees_getter_,
                unit_and_imports_->unit->sem_ir,
                GetImportedIRCount(unit_and_imports),
-               unit_and_imports_->unit->total_ir_count, vlog_stream) {}
+               unit_and_imports_->unit->total_ir_count, vlog_stream,
+               mangle_string_fingerprint) {}
 
 auto CheckUnit::Run() -> void {
   Timings::ScopedTiming timing(unit_and_imports_->unit->timings, "check");