[CMake] Check for C++14 instead of C++11

Now that LLVM moved to C++14, `COMPILER_RT_HAS_STD_CXX11_FLAG` should
become `COMPILER_RT_HAS_STD_CXX14_FLAG`.

I ran into this issue when replacing llvm::make_unique with
std::make_unique in an X-ray unit test. We are correctly passing
`-std=c++14`, but this got overwritten further down the invocation by
the compiler-rt flags. Given that this unit test is using LLVM headers,
this is bound to break sooner than later, regardless of my change.

Differential revision: https://reviews.llvm.org/D66271

llvm-svn: 368960