Replace assert by precondition in most places

[ahoppen]

This replaces assert statements by precondition statements in most cases. Afterwards I did a few performance measurements and reverted a few preconditions that had significant performance impact back to assert statements.

As it stands now, this regresses performance of parsing MovieSwiftUI by 2.9% (measured in number of instructions executed).

The biggest (>0.1%) performance regressions are coming from the following files. I’m open to debate if any of the preconditions in there should be changed to assert to improve performance but they all seem to provide good value at a reasonable cost to me

0.62% Sources/SwiftParser/TokenConsumer.swift
0.58% Sources/SwiftSyntax/Raw/RawSyntax.swift
0.52% Sources/SwiftParser/Lexer/Cursor.swift
0.41% Sources/SwiftSyntax/SyntaxText.swift
0.39% Sources/SwiftParser/TokenSpec.swift
0.29% Sources/SwiftParser/Recovery.swift
0.26% Sources/SwiftParser/Names.swift

I am also attaching the numbers spreadsheet that contains all performance measurements: Precondition performance measurements.zip

rdar://106874489

[ahoppen]

@swift-ci Please test