[Migrator] Suggest String <-> Substring conversions

Some APIs that expected a String now expect a Substring and vice
versa. To ease the transition, emit fix-its on conversion errors
between these types that the migrator can pick up.

When converting from Substring -> String, suggest wrapping in
`String.init`.

When converting from String -> Substring, suggest appending the
void subscript `[]`. (This isn't implemented yet so this is
hidden behind a flag).

This can possibly be generalized later when converting between
some sequence and its subsequence, such as Array and ArraySlice,
for example.

rdar://problem/31665649
rdar://problem/31666638

lib/Frontend/CompilerInvocation.cpp

@@ -884,6 +884,12 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                           const FrontendOptions &FrontendOpts) {
   using namespace options;
 
+  /// FIXME: Remove this flag when void subscripts are implemented.
+  /// This is used to guard preemptive testing for the fix-it.
+  if (Args.hasArg(OPT_fix_string_substring_conversion)) {
+    Opts.FixStringToSubstringConversions = true;
+  }
+
   if (auto A = Args.getLastArg(OPT_swift_version)) {
     auto vers = version::Version::parseVersionString(
       A->getValue(), SourceLoc(), &Diags);