[CMake] Rename `SANITIZER_HEADERS` to `SANITIZER_IMPL_HEADERS` under `lib/sanitizer_common`.

The variable name `SANITIZER_HEADERS` is already used for the list of
public headers in `include/CMakeLists.txt`.  Although the previous
implementation worked it's probably best to avoid shadowing global
variables to avoid confusion.

Differential Revision: https://reviews.llvm.org/D49176

llvm-svn: 336904