[BOLT][DWARF] Fix handling of GCed CU function

A CU can have only one function so CU will have low_pc/high_pc. If this funciton
is GCed by LLD low_pc will become 0x0, and BOLT can't map this to output. We
kind of were getting away with it in monolithic DWARF, but with split DWARF
there is only skeleton CU, so we end up with rnglist with header and array, but
no body. This caused LLDB to report an error.

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D151845