resolve: Replace Macros20NormalizedIdent with IdentKey

[petrochenkov]

This is a continuation of #150741 and #150982 based on the ideas from #151491 (comment).

Before this PR Macros20NormalizedIdent was used as a key in various "identifier -> its resolution" maps in rustc_resolve.
Macros20NormalizedIdent is a newtype around Ident in which SyntaxContext (packed inside Span) is guaranteed to be normalized using normalize_to_macros_2_0.
This type is also used in a number of functions looking up identifiers in those maps.
Macros20NormalizedIdent still contains span locations, which are useless and ignored during hash map lookups and comparisons due to Ident's special PartialEq and Hash impls.

This PR replaces Macros20NormalizedIdent with a new type called IdentKey, which contains only a symbol and a normalized unpacked syntax context. (E.g. IdentKey == Macros20NormalizedIdent minus span locations.)
So we avoid keeping additional data and doing some syntax context packing/unpacking.

Along with IdentKey you can often see orig_ident_span: Span being passed around.
This is an unnormalized span of the original Ident from which IdentKey was obtained.
It is not used in map keys, but it is used in a number of other scenarios:

• diagnostics
• edition checks
• allow_unstable checks

This is because normalize_to_macros_2_0 normalization is lossy and the normalized spans / syntax contexts no longer contain parts of macro backtraces, while the original span contains everything.

[petrochenkov]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 0b62e28 (0b62e28b1ffadafe2f62aaba7d06486bcdb96f83, parent: d222ddc4d90743dfc1e53b610be8fc9d95893d2c)

[rust-timer]

Finished benchmarking commit (0b62e28): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.1%, 0.3%]	2
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.2%]	21
Improvements ✅ (secondary)	-0.5%	[-1.4%, -0.1%]	24
All ❌✅ (primary)	-0.2%	[-0.3%, -0.2%]	21

Max RSS (memory usage)

Results (primary -1.7%, secondary -1.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.6%	[1.7%, 3.6%]	3
Improvements ✅ (primary)	-1.7%	[-1.8%, -1.7%]	2
Improvements ✅ (secondary)	-3.2%	[-6.4%, -2.3%]	5
All ❌✅ (primary)	-1.7%	[-1.8%, -1.7%]	2

Cycles

Results (primary 0.7%, secondary 5.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	5.8%	[4.9%, 6.8%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[0.7%, 0.7%]	1

Binary size

Results (secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	9
All ❌✅ (primary)	-	-	0

Bootstrap: 470.684s -> 470.503s (-0.04%)
Artifact size: 383.48 MiB -> 383.42 MiB (-0.02%)

[rustbot]

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[petrochenkov]

r? @nnethercote

[petrochenkov]

(I also want to try passing the ident: IdentKey, orig_ident_span: Span pair to resolve_ident_in_scope_set, but it may have some negative performance effects, so I'll try it separately.)