Reduce namespace pollution.

This leaves us with just the mspace stuff and three libdex functions to clean
up. We deliberately expose the JII API, and I don't think there's anything we
can really do about the art_..._from_code stuff (and at least that starts with
"art_").

Change-Id: I77e58e8330cd2afeb496642302dfe3311e68091a
diff --git a/src/compiler/codegen/CompilerCodegen.h b/src/compiler/codegen/CompilerCodegen.h
index d2e5f0a..f1ca07a 100644
--- a/src/compiler/codegen/CompilerCodegen.h
+++ b/src/compiler/codegen/CompilerCodegen.h
@@ -19,6 +19,8 @@
 
 #include "../CompilerIR.h"
 
+namespace art {
+
 /* Lower middle-level IR to low-level IR for the whole method */
 void oatMethodMIR2LIR(CompilationUnit* cUnit);
 
@@ -51,4 +53,6 @@
 /* Implemented in codegen/<target>/<target_variant>/ArchVariant.c */
 void oatGenMemBarrier(CompilationUnit* cUnit, int barrierKind);
 
+}  // namespace art
+
 #endif  // ART_SRC_COMPILER_COMPILERCODEGEN_H_