[mlir] Specify CMAKE_CXX_STANDARD to standalone dialect

This addresses a compilation failure on GCC 5:

error: #error This file requires compiler and library support for the
ISO C++ 2011 standard. This support must be enabled with the -std=c++11
or -std=gnu++11 compiler options.
 #error This file requires compiler and library support

Differential Revision: https://reviews.llvm.org/D79439