[cmake] Use relative cmake binary dir for processing pass plugins.

https://reviews.llvm.org/D61446 introduced a new function to process
pass plugins that used CMAKE_BINARY_DIR. This is problematic when LLVM
is a subproject. Instead use LLVM_BINARY_DIR to get the right relative
directory for cmake.

Patch by Alan Baker <alanbaker@google.com>

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D72109