Fully enable ExperimentalStringProcessing
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
