[CMake] Pass bugreport URL to standalone clang build

BUG_REPORT_URL is currently used both in LLVM and in Clang but declared
only in the latter.  This means that it's missing in standalone clang
builds and the driver ends up outputting:

  PLEASE submit a bug report to  and include [...]

(note the missing URL)

To fix this, include LLVM_PACKAGE_BUGREPORT in LLVMConfig.cmake
(similarly to how we pass PACKAGE_VERSION) and use it to fill
BUG_REPORT_URL when building clang standalone.

Differential Revision: https://reviews.llvm.org/D84987

(cherry picked from commit 21c165de)