SwiftRemoteMirror: Fix crash emitting capture descriptors involving implicit self

When emitting capture descriptors for functions with a smaller number of parameters
than SIL parameters, the compiler can crash indexing into the heap layout's element
types, because the capture index underflows to UINT_MAX.

rdar://problem/26404583

lib/IRGen/IRGenModule.h

@@ -629,8 +629,7 @@ public:
                                              CanSILFunctionType origCalleeType,
                                              CanSILFunctionType substCalleeType,
                                              ArrayRef<Substitution> subs,
-                                             HeapLayout &layout,
-                                             unsigned firstCaptureIndex);
+                                             HeapLayout &layout);
   std::string getBuiltinTypeMetadataSectionName();
   std::string getFieldTypeMetadataSectionName();
   std::string getAssociatedTypeMetadataSectionName();