Fix commonjs export= merging (#27368)

I'm surprised we haven't seen more of this; I suspect it's because the
mixed `module.exports=` + `export.foo=` pattern isn't that common.
However, it'll happen any time that the exported symbol is unknown;
getCommonJsExportEquals blithely clones unknownSymbol and proceeds to
stick the `exports.foo=` properties onto it.

This causes problems later, because the compiler checks for
unknownSymbol with `===`. The fix is to not stick properties onto a
clone of unknownSymbol. This makes the correct errors appear and removes
the crash.

src/compiler/checker.ts

@@ -2340,7 +2340,7 @@ namespace ts {
         }
 
         function getCommonJsExportEquals(exported: Symbol | undefined, moduleSymbol: Symbol): Symbol | undefined {
-            if (!exported || moduleSymbol.exports!.size === 1) {
+            if (!exported || exported === unknownSymbol || moduleSymbol.exports!.size === 1) {
                 return exported;
             }
             const merged = cloneSymbol(exported);