Optimizing: Improve const-string code generation.

For strings in the boot image, use either direct pointers
or pc-relative addresses. For other strings, use PC-relative
access to the dex cache arrays for AOT and direct address of
the string's dex cache slot for JIT.

For aosp_flounder-userdebug:
  - 32-bit boot.oat: -692KiB (-0.9%)
  - 64-bit boot.oat: -948KiB (-1.1%)
  - 32-bit dalvik cache total: -900KiB (-0.9%)
  - 64-bit dalvik cache total: -3672KiB (-1.5%)
    (contains more files than the 32-bit dalvik cache)
For aosp_flounder-userdebug forced to compile PIC:
  - 32-bit boot.oat: -380KiB (-0.5%)
  - 64-bit boot.oat: -928KiB (-1.0%)
  - 32-bit dalvik cache total: -468KiB (-0.4%)
  - 64-bit dalvik cache total: -1928KiB (-0.8%)
    (contains more files than the 32-bit dalvik cache)

Bug: 26884697
Change-Id: Iec7266ce67e6fedc107be78fab2e742a8dab2696
diff --git a/runtime/class_linker.h b/runtime/class_linker.h
index 886f586..b4b7f34 100644
--- a/runtime/class_linker.h
+++ b/runtime/class_linker.h
@@ -247,6 +247,12 @@
                                 Handle<mirror::DexCache> dex_cache)
       SHARED_REQUIRES(Locks::mutator_lock_);
 
+  // Find a String with the given index from the DexFile, storing the
+  // result in the DexCache if found. Return null if not found.
+  mirror::String* LookupString(const DexFile& dex_file, uint32_t string_idx,
+                               Handle<mirror::DexCache> dex_cache)
+      SHARED_REQUIRES(Locks::mutator_lock_);
+
   // Resolve a Type with the given index from the DexFile, storing the
   // result in the DexCache. The referrer is used to identity the
   // target DexCache and ClassLoader to use for resolution.