Add a stub constructor for LangOptions.

Having an out-of-line definition for the LangOptions constructor makes
it easier to enable experimental features one at a time for a build,
without rebuilding everything.

Author: DougGregor

include/swift/Basic/LangOptions.h

@@ -529,6 +529,8 @@ namespace swift {
       return ActiveConcurrencyModel == ConcurrencyModel::TaskToThread;
     }
 
+    LangOptions();
+
     /// Sets the target we are building for and updates platform conditions
     /// to match.
     ///

lib/Basic/LangOptions.cpp

@@ -29,6 +29,10 @@
 
 using namespace swift;
 
+LangOptions::LangOptions() {
+  // Note: Introduce default-on language options here.
+}
+
 struct SupportedConditionalValue {
   StringRef value;