[CMake] Copy C++ headers before configuring runtimes build

[CMake] Copy C++ headers before configuring runtimes build

Summary: On some platforms C++ headers are packaged with the compiler not the sysroot. If you don't copy C++ headers into the build include directory during configuraiton of the outer build the C++ check during the runtime configuration may get inaccurate results.

Reviewers: phosek, compnerd, smeenai, EricWF

Reviewed By: compnerd

Subscribers: EricWF, christof, libcxx-commits, mgorny, llvm-commits

Tags: #llvm, #libc

Differential Revision: https://reviews.llvm.org/D62155

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@361513 91177308-0d34-0410-b5e6-96231b3b80d8