Remove largely unused context from Visitor.
Track the visited AST's lifetime throughout Visitor.
auto merge of #17069 : eddyb/rust/visitor, r=pnkfelix

Few visitors used the context passing feature and it can be easily emulated.
The added lifetime threading allows a visitor to keep safe references to AST
nodes it visits, making a non-owning ast_map design possible, for #13316.
Fix bad error message copypasta
Add a Rust string ostream for LLVM
Add -C remark for LLVM optimization remarks

Fixes #17116.