Avoid use of std::string where we have const char*.

Removing the ClassHelper caused std::string creation for all calls to
Class::GetDescriptor and a significant performance regression. Make the
std::string an out argument so the caller can maintain it and its life time
while allowing GetDescriptor to return the common const char* case.

Don't generate GC maps when compilation is disabled.

Remove other uses of std::string that are occuring on critical paths.
Use the cheaper SkipClass in CompileMethod in CompilerDriver.
Specialize the utf8 as utf16 comparison code for the common shorter byte
encoding.
Force a bit of inlining, remove some UNLIKELYs (they are prone to pessimizing
code), add some LIKELYs.

x86-64 host 1-thread interpret-only of 57 apks:
Before: 29.539s
After: 23.467s

Regular compile:
Before: 1m35.347s
After: 1m20.056s

Bug: 16853450
Change-Id: Ic705ea24784bee24ab80084d06174cbf87d557ad
diff --git a/runtime/field_helper.cc b/runtime/field_helper.cc
index 40daa6d..5c85c46 100644
--- a/runtime/field_helper.cc
+++ b/runtime/field_helper.cc
@@ -41,17 +41,7 @@
 }
 
 const char* FieldHelper::GetDeclaringClassDescriptor() {
-  uint32_t field_index = field_->GetDexFieldIndex();
-  if (UNLIKELY(field_->GetDeclaringClass()->IsProxyClass())) {
-    DCHECK(field_->IsStatic());
-    DCHECK_LT(field_index, 2U);
-    // 0 == Class[] interfaces; 1 == Class[][] throws;
-    declaring_class_descriptor_ = field_->GetDeclaringClass()->GetDescriptor();
-    return declaring_class_descriptor_.c_str();
-  }
-  const DexFile* dex_file = field_->GetDexFile();
-  const DexFile::FieldId& field_id = dex_file->GetFieldId(field_index);
-  return dex_file->GetFieldDeclaringClassDescriptor(field_id);
+  return field_->GetDeclaringClass()->GetDescriptor(&declaring_class_descriptor_);
 }
 
 }  // namespace art