syntax: ABI-oblivious grammar
This PR has the following effects:
1. `extern $lit` is now legal where `$lit:literal` and `$lit` is substituted for a string literal.
2. `extern "abi_that_does_not_exist"` is now *syntactically* legal whereas before, the set of ABI strings was hard-coded into the grammar of the language. With this PR, the set of ABIs are instead validated and translated during lowering. That seems more appropriate.
3. `ast::FloatTy` is now distinct from `rustc_target::abi::FloatTy`. The former is used substantially more and the translation between them is only necessary in a single place.
4. As a result of 2-3, libsyntax no longer depends on librustc_target, which should improve pipe-lining somewhat.
cc @rust-lang/lang -- the points 1-2 slightly change the definition of the language but in a way which seems consistent with our general principles (in particular wrt. the discussions of turning things into semantic errors). I expect this to be uncontroversial but it's worth letting y'all know. :)
r? @varkor
We also sever syntax's dependency on rustc_target as a result.
This should slightly improve pipe-lining.
Moreover, some cleanup is done in related code.
It's a full conversion, except in `DefKey::compute_stable_hash()` where
a `Symbol` now is converted to an `InternedString` before being hashed.
This was necessary to avoid test failures.
Closure types have been moved to the namespace where they
are defined, and both closure and generator type names now
include the disambiguator.
This fixes an exception when lldb prints nested closures.
Fixes#57822
