Switch to use RSCompilerDriver.

This commit is large. It does:

1. Clean-up RSScript. ScriptCached and ScriptCompiled are removed.
2. Switch to use CompilerConfig.
3. Switch to use RSInfo. MCCacheReader and MCCacheWriter are removed.
4. Update C API implementation of libbcc (i.e., bcc.cpp). Note that
   it'll be completely removed once there's no clients relying on it.
diff --git a/lib/ExecutionEngine/BCCContext.cpp b/lib/ExecutionEngine/BCCContext.cpp
index 892fd5a..62995ed 100644
--- a/lib/ExecutionEngine/BCCContext.cpp
+++ b/lib/ExecutionEngine/BCCContext.cpp
@@ -19,7 +19,6 @@
 #include <new>
 
 #include "BCCContextImpl.h"
-#include "Compiler.h"
 #include "DebugHelper.h"
 #include "Source.h"
 
@@ -42,10 +41,7 @@
   GlobalContext = NULL;
 }
 
-BCCContext::BCCContext() : mImpl(new BCCContextImpl(*this)) {
-  // Initialize the LLVM compiler infrastructure.
-  Compiler::GlobalInitialization();
-}
+BCCContext::BCCContext() : mImpl(new BCCContextImpl(*this)) { }
 
 BCCContext::~BCCContext() {
   delete mImpl;