Not all runtimes can correctly operate with types that use noncopyable
generics. When the generic argument of a type is noncopyable, old
runtimes can't recognize that to correctly check conformances that may
be conditional on those arguments being Copyable, etc.
resolves rdar://126239335
If an expression refers to noncopyable storage, then default to performing
a borrowing switch, where `let` bindings in patterns borrow out of the
matched value. If an expression refers to a temporary value or explicitly
uses the `consume` keyword, then perform a consuming switch, where
`let` bindings take ownership of corresponding parts of the matched value.
Allow `_borrowing` to still be used to explicitly bind a pattern variable
as a borrow, with no-implicit-copy semantics for copyable values.
This prevents the move-only checker from trying to analyze the bindings
as partial consumptions, which ought to be unnecessary since SILGen
will always fully consume the subject as part of forming the bindings.
