AA metadata refactoring (introduce AAMDNodes)

In order to enable the preservation of noalias function parameter information
after inlining, and the representation of block-level __restrict__ pointer
information (etc.), additional kinds of aliasing metadata will be introduced.
This metadata needs to be carried around in AliasAnalysis::Location objects
(and MMOs at the SDAG level), and so we need to generalize the current scheme
(which is hard-coded to just one TBAA MDNode*).

This commit introduces only the necessary refactoring to allow for the
introduction of other aliasing metadata types, but does not actually introduce
any (that will come in a follow-up commit). What it does introduce is a new
AAMDNodes structure to hold all of the aliasing metadata nodes associated with
a particular memory-accessing instruction, and uses that structure instead of
the raw MDNode* in AliasAnalysis::Location, etc.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213859 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/MDBuilder.h

@@ -15,6 +15,7 @@
 #ifndef LLVM_IR_MDBUILDER_H
 #define LLVM_IR_MDBUILDER_H
 
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Support/DataTypes.h"
 #include <utility>
 
@@ -25,7 +26,6 @@ template <typename T> class ArrayRef;
 class LLVMContext;
 class MDNode;
 class MDString;
-class StringRef;
 
 class MDBuilder {
   LLVMContext &Context;