[ClangConfig] Copy ClangConfig.cmake to...

[ClangConfig] Copy ClangConfig.cmake to ${CMAKE_BINARY_DIR}/${CLANG_INSTALL_PACKAGE_DIR} instead of to ${CLANG_BINARY_DIR}/${CLANG_INSTALL_PACKAGE_DIR}.

This is an obvious bug since ClangConfig.cmake looks for ClangTargets.cmake in
${CMAKE_CURRENT_LIST_DIR}. But ClangTargets.cmake is in
${CMAKE_BINARY_DIR}/${CLANG_INSTALL_PACKAGE_DIR}, so it will always fail with an
in tree build.

In the case where clang is built out of tree, this is still correct since
CMAKE_BINARY_DIR and CLANG_BINARY_DIR will be the same.

llvm-svn: 274168