[libcxx] Do not implicitly #include assert.h

Users should only get the assert() macros if they explicitly include
them.

Found after switching from the GNU C++ stdlib to the LLVM C++ stdlib.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@372963 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed