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

lib/Frontend/CompilerInvocation.cpp

@@ -542,8 +542,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     // forward slash regex `/.../`.
     if (!Opts.EnableExperimentalStringProcessing)
       Opts.EnableBareSlashRegexLiterals = false;
-  } else {
-    Opts.EnableExperimentalStringProcessing = true;
   }
 
   Opts.DisableAvailabilityChecking |=