Combining unknown type+value symbol->unknownSymbol

Previously, when getExternalModuleMember tried to combine unknownSymbol
from a type and unknownSymbol from a value, it combineValueAndTypeSymbol
would create a new franken-symbol that was no longer equal to
unknownSymbol.

src/compiler/checker.ts

@@ -1364,6 +1364,9 @@ namespace ts {
         // An 'import { Point } from "graphics"' needs to create a symbol that combines the value side 'Point'
         // property with the type/namespace side interface 'Point'.
         function combineValueAndTypeSymbols(valueSymbol: Symbol, typeSymbol: Symbol): Symbol {
+            if (valueSymbol === unknownSymbol && typeSymbol === unknownSymbol) {
+                return unknownSymbol;
+            }
             if (valueSymbol.flags & (SymbolFlags.Type | SymbolFlags.Namespace)) {
                 return valueSymbol;
             }