Mark C++ reference parameters as dereferenceable

Because references must be initialized using some evaluated expression, they
must point to something, and a callee can assume the reference parameter is
dereferenceable. Taking advantage of a new attribute just added to LLVM, mark
them as such.

Because dereferenceability in addrspace(0) implies nonnull in the backend, we
don't need both attributes. However, we need to know the size of the object to
use the dereferenceable attribute, so for incomplete types we still emit only
nonnull.

llvm-svn: 213386
diff --git a/clang/test/CodeGenCXX/dllimport.cpp b/clang/test/CodeGenCXX/dllimport.cpp
index 93b5c82..59f8f63 100644
--- a/clang/test/CodeGenCXX/dllimport.cpp
+++ b/clang/test/CodeGenCXX/dllimport.cpp
@@ -539,11 +539,11 @@
   // MO1-DAG: @"\01?b@T@@2HA" = external dllimport global i32
 
   T& operator=(T&) = default;
-  // MO1-DAG: define available_externally dllimport x86_thiscallcc nonnull %struct.T* @"\01??4T@@QAEAAU0@AAU0@@Z"
+  // MO1-DAG: define available_externally dllimport x86_thiscallcc dereferenceable({{[0-9]+}}) %struct.T* @"\01??4T@@QAEAAU0@AAU0@@Z"
 
   T& operator=(T&&) = default;
   // Note: Don't mark inline move operators dllimport because current MSVC versions don't export them.
-  // MO1-DAG: define linkonce_odr x86_thiscallcc nonnull %struct.T* @"\01??4T@@QAEAAU0@$$QAU0@@Z"
+  // MO1-DAG: define linkonce_odr x86_thiscallcc dereferenceable({{[0-9]+}}) %struct.T* @"\01??4T@@QAEAAU0@$$QAU0@@Z"
 };
 USEMEMFUNC(T, a)
 USEVAR(T::b)