Added atomic_fetch_min, max, umin, umax intrinsics to clang.

These intrinsics work exactly as all other atomic_fetch_* intrinsics and allow to create *atomicrmw* with ordering.
Updated the clang-extensions document.

Differential Revision: https://reviews.llvm.org/D46386

llvm-svn: 332193
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index f3338dc..b2593ef 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -4052,6 +4052,8 @@
   case AO__atomic_or_fetch:
   case AO__atomic_xor_fetch:
   case AO__atomic_nand_fetch:
+  case AO__atomic_fetch_min:
+  case AO__atomic_fetch_max:
     return 3;
 
   case AO__opencl_atomic_store: