Fix transitive whole_static_libs on prebuilt static libraries

whole_static_libs on static library usually uses the .o files from
the dependency, but on a prebuilt static library has to use the .a file.
whole_static_libs on a static library that itself has whole_static_libs
on a prebuilt static library also needs to use the .a file transitively.

Test: TestWholeStaticLibPrebuilts
Change-Id: I9d4aabf72be0941b461a72e610f2b28287251cb2
diff --git a/cc/linkable.go b/cc/linkable.go
index 02d7047..d4b4770 100644
--- a/cc/linkable.go
+++ b/cc/linkable.go
@@ -352,6 +352,11 @@
 	Objects       Objects
 	ReuseObjects  Objects
 
+	// A static library may contain prebuilt static libraries included with whole_static_libs
+	// that won't appear in Objects.  They are transitively available in
+	// WholeStaticLibsFromPrebuilts.
+	WholeStaticLibsFromPrebuilts android.Paths
+
 	// This isn't the actual transitive DepSet, shared library dependencies have been
 	// converted into static library analogues.  It is only used to order the static
 	// library dependencies that were specified for the current module.