Sema: Clean up handling of protocol operators with concrete operands

[slavapestov]

More peeling off refactorings from #28698.

In this case we would "devirtualize" the protocol requirement call
by building the AST to model a direct reference to the witness.

Previously this was done by recursively calling typeCheckExpression(),
but the only thing this did was recover the correct substitutions
for the call.

Instead, we can just build the right SubstitutionMap directly.
Unfortunately, while we serialize enough information in the AST
to devirtualize calls at the SIL level, we do not for AST Exprs.

This is because SIL devirtualization builds a reference to the
witness thunk signature, which is an intermediate step between
the protocol requirement and the witness. I get around this by
deriving the substitutions from walking in parallel over the
interface type of the witness, together with the inferred type
of the call expression.

[slavapestov]

@DougGregor @xedin This eliminates the second-to-last typeCheckExpression() call from inside CSApply. The last one is in the handling of EditorPlaceholderExpr.

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

@swift-ci Please test source compatibility

[slavapestov]

@swift-ci Please test source compatibility Release

[slavapestov]

@swift-ci Please benchmark

[slavapestov]

@swift-ci Please test source compatibility Release

[swift-ci]

Performance: -O

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
FlattenListLoop	2654	3136	+18.2%	0.85x (?)

Code size: -Osize

Performance: -Onone

Code size: -swiftlibs

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the
regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false
alarms. Unexpected regressions which are marked with '(?)' are probably noise.
If you see regressions which you cannot explain you can try to run the
benchmarks again. If regressions still show up, please consult with the
performance team (@eeckstein).

Hardware Overview

  Model Name: Mac mini
  Model Identifier: Macmini8,1
  Processor Name: Intel Core i7
  Processor Speed: 3.2 GHz
  Number of Processors: 1
  Total Number of Cores: 6
  L2 Cache (per Core): 256 KB
  L3 Cache: 12 MB
  Memory: 64 GB