This became necessary after recent function type changes that keep
substituted generic function types abstract even after substitution to
correctly handle automatic opaque result type substitution.
Instead of performing the opaque result type substitution as part of
substituting the generic args the underlying type will now be reified as
part of looking at the parameter/return types which happens as part of
the function convention apis.
rdar://62560867
Move the source files from ParserSIL into the SIL library and remove the
ParserSIL library. ParsersSIL doesn't need to be its own library and this change will
simplify our builds.
