runtime: Bitstring implementation for subtype checking (2/4).

Implement the subtype checking label as a stand-alone data type.

This stores the bitstring and overflow bits. The bitstring
contains the encoded (path to root, next) bits.

Test: art/test.py -b -j32 --host --target
Bug: 64692057
Change-Id: I2229e1938f5377d637595ba3f81b8af3464d1ae7
diff --git a/runtime/Android.bp b/runtime/Android.bp
index 3059533..0e9e989 100644
--- a/runtime/Android.bp
+++ b/runtime/Android.bp
@@ -604,6 +604,7 @@
         "prebuilt_tools_test.cc",
         "reference_table_test.cc",
         "runtime_callbacks_test.cc",
+        "subtype_check_info_test.cc",
         "thread_pool_test.cc",
         "transaction_test.cc",
         "type_lookup_table_test.cc",