Fix crash on nested interpolation

[omochi]

Fix #618, #636

Lex.Cursor.skipToEndOfInterpolatedExpression is called from two paths.

One is from Lexer.Cursor.lexStringLiteral.
Another is from Lexer.lexToEndOfInterpolatedExpression from Parser.parseStringLiteralSegments.

In first case, cursor position is after of ( which opens interpolation.
But skipToEndOfInterpolatedExpression expects it called on position of (.

So Lexer split tokens wrongly with some inputs.
For example:

"text \(a.m({ "\($0)" }).j(s: ",")) text"
                       ^(1)       ^(2)

Lexer closes interpolation (1) wrongly. (2) is correct.

To fix this issue, I change lexStringLiteral.
As changing advance to peek,
cursor position is on ( when it calls skipToEndOfInterpolatedExpression.

[ahoppen]

Could you add test cases for the two fixed issues as described in https://github.com/apple/swift-syntax/blob/main/Sources/SwiftParser/SwiftParser.docc/FixingBugs.md#round-trip-failure-or-parser-crash?

[omochi]

@ahoppen I added test cases 😀

[ahoppen]

Thanks for fixing this crash 👍

[ahoppen]

@swift-ci Please test