IRGen: GenericContextScope always resets CurGenericSignature to old value

Previously, we didn't reset it if the old value was null, however this
enabled a very fragile anti-pattern:

    // ... CurGenericSignature is not set here ...
    {
        GenericContextScope scope(IGM, newSig);

        // signature is now newSig
        ...
    }

    // ... old signature is still newSig!
    foo(IGM.getCurGenericContext()); // works, most of the time

Author: slavapestov

lib/IRGen/GenType.cpp

@@ -1441,9 +1441,6 @@ TypeConverter::~TypeConverter() {
 }
 
 void TypeConverter::setGenericContext(CanGenericSignature signature) {
-  if (!signature)
-    return;
-  
   CurGenericSignature = signature;
 
   // Clear the dependent type info cache since we have a new active signature

lib/IRGen/GenType.h

@@ -277,8 +277,6 @@ class GenericContextScope {
   {}
 
   ~GenericContextScope() {
-    if (!newSig)
-      return;
     assert(TC.CurGenericSignature == newSig);
     TC.setGenericContext(oldSig);
   }