Avoid indent for single-line argument lists in BasicFormat

[broken-circle]

Closes #1473.
Closes #2082.

FunctionCallExprSyntax.arguments and TupleExprSyntax.elements were unconditional indentation scopes in BasicFormat.requiresIndent. When the argument list itself was single-line (no argument starting on its own line) but contained a multi-line expression like a closure or collection literal, the inner expression's indentation got double-counted: once from the argument-list scope and once from the expression's own scope.

For example:

foo(someClosure: { _ in
    return 1
})

formatted to:

foo(someClosure: { _ in
        return 1
    })

This PR treats the argument-list keypaths as indentation scopes only when the argument list is multi-line at the argument level. That indentation only exists to lay out arguments that wrap onto their own lines. When arguments are all on the same line as (, any multi-line content comes from a nested expression that brings its own scope.

The same change is applied to TupleExprSyntax.elements, which has the same structure (also a LabeledExprListSyntax), with the same bug (see updated ExpressionMacroTests.testStringifyExpression()).

Testing

This PR adds three new tests in BasicFormatTests, and updates 5 existing tests in CallToTrailingClosuresTest, VariableTests, DebugDescriptionTests, and ExpressionMacroTests to expect the correct format.