[Tablegen] Replace uses of formatted_raw_ostream with raw_ostream in the predicate expander. NFCI

This is a follow-up of r339552.

As pointed out by Craig in D50566, we don't need a formatted_raw_ostream to
indent strings. We can use instead raw_ostream::indent().

Internally, class PredicateExpander already keeps track of the current
indentation level. Also, the grammar for predicates is well parenthesized, and
therefore we don't need to use a formatted_raw_ostream to continuously track the
column number. Instead we can safely replace all the uses of
formatted_raw_ostream::PadToColumn() with uses of raw_ostream::indent().

By replacing formatted_raw_ostream with a simpler raw_ostream, we also avoid the
implicit check on the newline character on every print to stream.

No functional change intended.

llvm-svn: 339577