Add "-swift-version <n>" that sets LangOpts.EffectiveLanguageVersion.

This flag switches the "effective language version" of the compiler,
at least to any version supported (as of this change: "3" or "3.0").

At the moment nothing uses it except the language version build
configuration statements (#if swift(...)) and various other places
that report, encode, or otherwise check version numbers.

In the future, it's intended as scaffolding for backwards compatibility.

Fixes SR-2582

lib/Frontend/CompilerInvocation.cpp

@@ -765,6 +765,18 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                           const FrontendOptions &FrontendOpts) {
   using namespace options;
 
+  if (auto A = Args.getLastArg(OPT_swift_version)) {
+    auto vers = version::Version::parseVersionString(
+      A->getValue(), SourceLoc(), &Diags);
+    if (vers.hasValue() &&
+        vers.getValue().isValidEffectiveLanguageVersion()) {
+      Opts.EffectiveLanguageVersion = vers.getValue();
+    } else {
+      Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
+                     A->getAsString(Args), A->getValue());
+    }
+  }
+
   Opts.AttachCommentsToDecls |= Args.hasArg(OPT_dump_api_path);
 
   Opts.UseMalloc |= Args.hasArg(OPT_use_malloc);