Fix data race when ALLOW_MISSING_DEPENDENCIES=true

Missing dependencies were modifying a slice owned by a dependency,
which could cause a data race.  Return a copy instead.

Test: build with race detector enabled
Bug: 70706626
Change-Id: I9f68c887bee94348f3e11284c0833d2e04d62599

cc/library.go

@@ -709,7 +709,7 @@ func (library *libraryDecorator) buildShared() bool {
 }
 
 func (library *libraryDecorator) getWholeStaticMissingDeps() []string {
-	return library.wholeStaticMissingDeps
+	return append([]string(nil), library.wholeStaticMissingDeps...)
 }
 
 func (library *libraryDecorator) objs() Objects {