resolve/hygiene: `macro_rules` are not "legacy"
The "modern" vs "legacy" naming was introduced by jseyfried during initial implementation of macros 2.0.
At this point it's clear that `macro_rules` are not going anywhere and won't be deprecated in the near future.
So this PR changes the naming "legacy" (when it implies "macro_rules") to "macro_rules".
This should also help people reading this code because it's wasn't obvious that "legacy" actually meant "macro_rules" in these contexts.
The most contentious renaming here is probably
```
fn modern -> fn normalize_to_macros_2_0
fn modern_and_legacy -> fn normalize_to_macro_rules
```
Other alternatives that I could think of are `normalize_to_opaque`/`normalize_to_semitransparent`, or `strip_non_opaque`/`strip_transparent`, but they seemed less intuitive.
The documentation to these functions can be found in `symbol.rs`.
r? @matthewjasper
ast: `Mac`/`Macro` -> `MacCall`
It's now obvious that these refer to macro calls rather than to macro definitions.
It's also a single name instead of two different names in different places.
`rustc_expand` usually calls macro calls in a wide sense (including attributes and derives) "macro invocations", but structures and variants renamed in this PR are only relevant to fn-like macros, so it's simpler and clearer to just call them calls.
cc https://github.com/rust-lang/rust/pull/63586#discussion_r314232513
r? @eddyb
Improve expression & attribute parsing
This PR includes misc improvements to expression and attribute parsing.
1. Some code simplifications
2. Better recovery for various block forms, e.g. `loop statements }` (missing `{` after `loop`). (See e.g., `block-no-opening-brace.rs` among others for examples.)
3. Added recovery for e.g., `unsafe $b` where `$b` refers to a `block` macro fragment. (See `bad-interpolated-block.rs` for examples.)
4. ^--- These are done so that code sharing in block parsing is increased.
5. Added recovery for e.g., `'label: loop { ... }` (See `labeled-no-colon-expr.rs`.)
6. Added recovery for e.g., `&'lifetime expr` (See `regions-out-of-scope-slice.rs`.)
7. Added recovery for e.g., `fn foo() = expr;` (See `fn-body-eq-expr-semi.rs`.)
8. Simplified attribute parsing code & slightly improved diagnostics.
9. Added recovery for e.g., `Box<('a) + Trait>`.
10. Added recovery for e.g, `if true #[attr] {} else #[attr] {} else #[attr] if true {}`.
r? @estebank
