[libc++abi] Limit libc++ header search to specified paths

Right now, when libc++abi is locating libc++ headers, it specifies
several search locations, but it also doesn't prevent CMake from looking
for those headers in system directories. I don't know if this was
intentional or an oversight, but it has several issues:

* We're looking specifically for the vector header, which could just as
  easily be found in a libstdc++ (or other C++ library) installation.
* No system I know of places their C++ headers directly in system
  include directories (they're always under a C++ subdirectory), so the
  system search will never succeed.
* find_path searches system paths before the user-specified PATHS, so
  if some system does happen to have C++ headers in its system include
  directories, those headers will be preferred, which doesn't seem
  desirable.

It makes sense to me to limit this header search to the explicitly
specified paths (using NO_DEFAULT_PATH, as is done for the other
find_path call in this file), but I'm putting it up for review in case
there's some use case I'm not thinking of.

Differential Revision: https://reviews.llvm.org/D48694

llvm-svn: 336032

libcxxabi/CMakeLists.txt

@@ -116,6 +116,7 @@ find_path(
         ${CMAKE_BINARY_DIR}/${LIBCXXABI_LIBCXX_INCLUDES}
         ${LIBCXXABI_LIBCXX_INCLUDE_DIRS}
         ${LLVM_INCLUDE_DIR}/c++/v1
+  NO_DEFAULT_PATH
   NO_CMAKE_FIND_ROOT_PATH
   )