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/utf.h b/runtime/utf.h
index 4abd605..27d2fd5 100644
--- a/runtime/utf.h
+++ b/runtime/utf.h
@@ -83,6 +83,7 @@
 int32_t ComputeUtf16Hash(mirror::CharArray* chars, int32_t offset, size_t char_count)
     SHARED_REQUIRES(Locks::mutator_lock_);
 int32_t ComputeUtf16Hash(const uint16_t* chars, size_t char_count);
+int32_t ComputeUtf16HashFromModifiedUtf8(const char* utf8, size_t utf16_length);
 
 // Compute a hash code of a modified UTF-8 string. Not the standard java hash since it returns a
 // uint32_t and hashes individual chars instead of codepoint words.