[X86] Add ENQCMD instructions

For more details about these instructions, please refer to the latest
ISE document:
https://software.intel.com/en-us/download/intel-architecture-instruction-set-extensions-programming-reference.

Patch by Tianqing Wang (tianqing)

Differential Revision: https://reviews.llvm.org/D62282

llvm-svn: 362685
diff --git a/clang/docs/ClangCommandLineReference.rst b/clang/docs/ClangCommandLineReference.rst
index ee58c2d..5a34c96 100644
--- a/clang/docs/ClangCommandLineReference.rst
+++ b/clang/docs/ClangCommandLineReference.rst
@@ -2657,6 +2657,8 @@
 
 .. option:: -mcx16, -mno-cx16
 
+.. option:: -menqcmd, -mno-enqcmd
+
 .. option:: -mf16c, -mno-f16c
 
 .. option:: -mfma, -mno-fma