ASTGen always builds with the host Swift compiler, without requiring
bootstrapping, and is enabled in more places. Move the regex literal
parsing logic there so it is enabled in more host environments, and
makes use of CMake's Swift support. Enable all of the regex literal
tests when ASTGen is built, to ensure everything is working.
Remove the "AST" and "Parse" Swift modules from SwiftCompilerSources,
because they are no longer needed.
Previously we would only enable by default when
`parseArgs` was called. However this wouldn't
enable it for clients such as LLDB, who provide
their own invocation. Switch the default to `true`
in the `LangOptions`, and remove some redundant
uses of `-enable-experimental-string-processing`.
The frontend flag remains, as it may be useful to
disable.
rdar://107419385
rdar://101765556
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.
We add a new flag to disable the implicit import of `_StringProcessing`, similar to `-disable-implicit-concurrency-module-import`. We need this to build `_RegexParser` when `-enable-experimental-string-processing` is enabled by default, because `_StringProcessing` currently imports `_RegexParser` publicly (non-implementation-only).
