Initialize cpp_mangle_context_ in Mangler's constructor

[bricknerb]

This is a followup of a comment in #5062.

Add a mutable AST pointer to FileContext.

This is necessary since we use Clang with lack of const correctness.

Alternatives in Clang:

• Change ASTUnit::getASTContext() const to return a non-const ASTContext. Tried and was rejected upstream due to weakening const correctness.
• Change createMangleContext() to be const. Tried that and it seems like it relies heavily on non const API.
• Change MangleContext::mangleName() to const. Tried that but there are several lazy initialization and id creations happening that modify the context. See details in [Clang] Make MangleContext::mangleMSGuidDecl(), MangleContext::mangleObjCMethodName() and MangleContext::mangleObjCMethodNameAsSourceName() const methods llvm/llvm-project#130613.

Alternatives in Carbon:

• Use const_cast on ASTContext when calling createMangleContext().
• Make FileContext::sem_ir_ point to a mutable SemIR::File.
• Change File::cpp_ast() to be const while keeping it return a mutable pointer.

Part of #4666.

[jonmeow]

I share the same concerns regarding a const accessor returning a non-const value as the clang owners gave you. If it were how the API were intended to behave, that'd be one thing, but I don't think we should work around that in our code.

Going on the advice from clang owners, you could const_cast with a TODO. But maybe it would make more sense to pass the mutable AST straight to lowering, rather than trying to pull it off the const SemIR?

[bricknerb]

I share the same concerns regarding a const accessor returning a non-const value as the clang owners gave you. If it were how the API were intended to behave, that'd be one thing, but I don't think we should work around that in our code.

Going on the advice from clang owners, you could const_cast with a TODO. But maybe it would make more sense to pass the mutable AST straight to lowering, rather than trying to pull it off the const SemIR?

ok, SG, passing a mutable AST to lowering now.