Add a dereferenceable attribute

This attribute indicates that the parameter or return pointer is
dereferenceable. Practically speaking, loads from such a pointer within the
associated byte range are safe to speculatively execute. Such pointer
parameters are common in source languages (C++ references, for example).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213385 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm-c/Core.h

@@ -168,6 +168,7 @@ typedef enum {
     LLVMInAllocaAttribute = 1ULL << 36,
     LLVMNonNullAttribute = 1ULL << 37,
     LLVMJumpTableAttribute = 1ULL << 38,
+    LLVMDereferenceableAttribute = 1ULL << 39,
     */
 } LLVMAttribute;