[ThinLTO] only emit used or referenced CFI records to index

Summary: We emit CFI_FUNCTION_DEFS and CFI_FUNCTION_DECLS to
distributed ThinLTO indices to implement indirect function call
checking.  This change causes us to only emit entries for functions
that are either defined or used by the module we're writing the index
for (instead of all functions in the combined index), which can make
the indices substantially smaller.

Fixes PR42378.

Reviewers: pcc, vitalybuka, eugenis

Subscribers: mehdi_amini, hiraditya, dexonsmith, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63887

llvm-svn: 365537