Initialize cpp_mangle_context_ in Mangler's constructor

This is a followup of [a comment](https://github.com/carbon-language/carbon-lang/pull/5062/files/89e56d51858bcc18d4242d4e5c9ee0e7496d887e#r1979993815) in carbon-language#5062.

Change `File::cpp_ast()` to be `const` while keeping it returning a non-const pointer to `clang::ASTUnit`.
This makes the fact we use [Clang with lack of const correctness](llvm/llvm-project#130096 (comment)) explicit.

Alternatives:
* Change `ASTUnit::getASTContext() const` to return a non-const `ASTContext`. [Rejected due to weakening const correctness](llvm/llvm-project#130096).
* Change `createMangleContext()` to be `const`. Tried that and it seems like it relies heavily on non const API.
* Change Clang `MangleContext::mangleName()` to `const`.
* Use `const_cast` on `ASTContext` when calling `createMangleContext()`.

Author: bricknerb

toolchain/lower/mangler.cpp

@@ -162,13 +162,9 @@ auto Mangler::Mangle(SemIR::FunctionId function_id,
 }
 
 auto Mangler::MangleCppClang(const clang::NamedDecl* decl) -> std::string {
-  if (!cpp_mangle_context_) {
-    // We assume all declarations are from the same AST Context.
-    // TODO: Consider initializing this in the constructor. This is related to:
-    // https://github.com/carbon-language/carbon-lang/issues/4666. See
-    // https://github.com/carbon-language/carbon-lang/pull/5062/files/89e56d51858bcc18d4242d4e5c9ee0e7496d887e#r1979993815
-    cpp_mangle_context_.reset(decl->getASTContext().createMangleContext());
-  }
+  CARBON_CHECK(
+      cpp_mangle_context_,
+      "Mangling of a C++ imported declaration without a Clang `MangleContext`");
 
   RawStringOstream cpp_mangled_name;
   cpp_mangle_context_->mangleName(decl, cpp_mangled_name);

toolchain/lower/mangler.h

@@ -22,7 +22,14 @@ class Mangler {
  public:
   // Initialize a new Mangler instance for mangling entities within the
   // specified `FileContext`.
-  explicit Mangler(FileContext& file_context) : file_context_(file_context) {}
+  explicit Mangler(FileContext& file_context)
+      : file_context_(file_context),
+        cpp_mangle_context_(file_context.sem_ir().cpp_ast()
+                                ? file_context.sem_ir()
+                                      .cpp_ast()
+                                      ->getASTContext()
+                                      .createMangleContext()
+                                : nullptr) {}
 
   // Produce a deterministically unique mangled name for the function specified
   // by `function_id` and `specific_id`.

toolchain/sem_ir/file.h

@@ -187,7 +187,7 @@ class File : public Printable<File> {
   auto import_cpps() const -> const ValueStore<ImportCppId>& {
     return import_cpps_;
   }
-  auto cpp_ast() -> clang::ASTUnit* { return cpp_ast_; }
+  auto cpp_ast() const -> clang::ASTUnit* { return cpp_ast_; }
   // TODO: When the AST can be created before creating `File`, initialize the
   // pointer in the constructor and remove this function.
   auto set_cpp_ast(clang::ASTUnit* cpp_ast) -> void { cpp_ast_ = cpp_ast; }