Only trim the trailing newline of a %L argument - #2391

The %L trim only affects layout. Both callers manage the final newline themselves: ensureEndsWithNewLine() makes the kdoc end in exactly one before */, and PropertySpec writes its own after trimTrailingNewLine(). Trailing newlines on a String passed to %L collide with that newline: addKdoc("%L", "text\n") would get an empty * line before */ without the trim, and initializer("%L", "foo()\n") would be followed by a blank line. Neither changes the generated program. For %S and %P the newline is part of the literal's value, so trimming it changed the output's meaning, which is why #2391 removed the trim there and kept it for %L.

Never trimming is the cleaner rule and I would take it. A CodeBlock passed to %L was never trimmed, so dropping the String branch applies one rule to every argument: the library emits what it is given, and a caller who passes %L a string ending in a newline trims it themselves, as %S and %P callers now do. The cost is the two blank lines above appearing for existing callers who relied on the trim.