[SourceKit/CodeFormat] Fix multi-line array literal elements not indenting relative to their first line.

[nathawes]

Previously each item in an array literal wasn't considered to be an indent context, so we under-indented their continuation lines as in the example below:

doStuffWithList([
    firstItem
    .map {$0}       // This line should be indented further.
    .append(\.foo),   // And so should this one.
    secondItem
    .filter {$0 > 5}
])

After this change, each item is an indent context so we get the below:

doStuffWithList([
    firstItem
        .map {$0}
        .append(\.foo),
    secondItem
        .filter {$0 > 5}
])

Resolves rdar://problem/64834040

[nathawes]

@swift-ci please test

[nathawes]

@swift-ci please test