Because we don't form a type-checked call to the
Regex initializer in the AST, we need to explicitly
handle the availability checking for `Regex<Output>`
and the initializer we're implicitly calling.
rdar://92156542
Friend PR: apple/swift-experimental-string-processing#282.
Also remove the `-enable-experimental-pairwise-build-block` flag when building regex modules as the feature is already on by default.
