Escapingness is a property of the type of a value, not a property of a function
parameter. Having it as a separate parameter flag just meant one more piece of
state that could get out of sync and cause weird problems.
Instead, always look at the noescape bit in a function type as the canonical
source of truth.
This does mean that '@escaping' is now printed in a few diagnostics where it was
not printed before; we can investigate these as separate issues, but it is
correct to print it there because the function types in question are, in fact,
escaping.
Fixes <https://bugs.swift.org/browse/SR-10256>, <rdar://problem/49522774>.
As for `OpenExistentialExpr`, normal ASTWalker walks into the existential
expression first, then walks into sub expression. However,
`SourceEntitiyWalker` must walk AST tree by source order. Handle
`OpenExistentialExpr` and its `OpaqueValueExpr` so that
`SourceEntityWalker` walks to them in "outer to inner" manner.
rdar://problem/41147733
