SyntaxTree: Add jl_source attribute for debugging JuliaLowering

[mlechu]

Given that @ast is the main way we build trees, we can do a neat trick: store the calling line in the tree it created. I've been using this locally, and it's been helpful for debugging.

Also refactor our use of ensure_attributes so new ones don't need to be added in many separate places.

[topolarity]

Nice utility!

Should this be put behind a compile-time flag, so that we can disable it for profiling / public releases?

[mlechu]

Sure, though I'd like to not require a debug build to use it at the moment. Is there an existing flag I can use, or should I make a lowering-specific one?

[topolarity]

Sure, though I'd like to not require a debug build to use it at the moment. Is there an existing flag I can use, or should I make a lowering-specific one?

Yep, fine by me if it's on almost all the time for now.

There is Core.Compiler.is_asserts(), but it sounds like you'll want a lowering-specific flag.