[llvm] [cmake] Export LLVM_ENABLE_NEW_PASS_MANAGER into LLVMConfig.cmake

Include the vaue of LLVM_ENABLE_NEW_PASS_MANAGER in generated
LLVMConfig.cmake since it is needed by clang's build system.  This fixes
test failures when the new pass manager is enabled (i.e. by default)
by having clang's CMake files correctly detect that and skip relevant
tests.

Differential Revision: https://reviews.llvm.org/D107628

(cherry picked from commit 889a1e69)