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).
