[CMake] Move BUG_REPORT_URL from clang to llvm.

It was too late to set BUG_REPORT_URL after configure_file(config.h).
BUG_REPORT_URL in config.h.cmake would be updated at 2nd run of cmake.
It caused many recompilations.

FYI, configure handles BUG_REPORT_URL in llvm side.

llvm-svn: 199076