[YAMLIO] Remove trailing spaces when outputting maps

llvm::yaml::Output::paddedKey unconditionally outputs spaces, which
are superfluous if the value to be dumped is a sequence or map.
Change `bool NeedsNewLine` to `StringRef Padding` so that it can be
overridden to `\n` if the value is a sequence or map.

An empty map/sequence is special. It is printed as `{}` or `[]` without
a newline, while a non-empty map/sequence follows a newline. To handle
this distinction, add another variable `PaddingBeforeContainer` and does
the special handling in endMapping/endSequence.

Reviewed By: grimar, jhenderson

Differential Revision: https://reviews.llvm.org/D64566

llvm-svn: 365869