Always parse macro expansions, regardless of language mode, and
eliminate the fallback path for very, very, very old object literals
like `#Color`. Instead, check for the feature flag for macro
declaration and at macro expansion time, since this is a semantic
restriction.
While here, refactor things so the vast majority of the macro-handling
logic still applies even if the Swift Swift parser is disabled. Only
attempts to expand the macro will fail. This allows us to enable the
macro-diagnostics test everywhere.
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.