Sema: Fixes for partial application of protocol methods

Previously the placement of the OpenExistentialExpr was determined
entirely from the natural argument count of the function.

There was a hack to add any missing OpenExistentialExprs at the top
level, but this didn't work if the method had a Self return value
and there were intermediate expressions, eg, if someMethod has a
Self return, foo(anExistential.someMethod) would generate a
diagnostic about open existentials.

Change ExprRewriter to use a new existential placement algorithm
that instead walks up the expression stack to determine the outermost
function application of an existential base, and insert the
OpenExistentialExpr there.

Progress on <rdar://problem/21289579>.

Swift SVN r29448

Author: slavapestov