[StripDeadDebugInfo] Drop dead CUs for const global expression

Compile units are not left explicitly if they have const global
expression.  They should be left if that const global is used in a
function that is contained in extracted module.

Signed-off-by: Mikhail Lychkov <mikhail.lychkov@intel.com>

Differential Revision: https://reviews.llvm.org/D131179