Have clang generate full debug info for libvulkan

Apply -fstandalone-debug cflag to avoid incomplete types in debug_info
for libvulkan.so

Test: make
Change-Id: I9a577aba2f25a6284bc6a4b39e5da5b63a15e93a
diff --git a/vulkan/libvulkan/Android.bp b/vulkan/libvulkan/Android.bp
index 7eaf7b2..fed8481 100644
--- a/vulkan/libvulkan/Android.bp
+++ b/vulkan/libvulkan/Android.bp
@@ -39,6 +39,9 @@
         "-Wno-switch-enum",
         "-Wno-undef",
 
+        // Have clang emit complete debug_info.
+        "-fstandalone-debug",
+
         //"-DLOG_NDEBUG=0",
     ],