[GlobalMerge] Ensure that the MustKeepGlobalVariables has all globals from each landi...

[GlobalMerge] Ensure that the MustKeepGlobalVariables has all globals from each landingpad clause.

The filter clause in the landingpad may not have a GlobalVariable operand.
It may instead have a ConstantArray of operands and each operand within this
ConstantArray should also be checked to see if it is a GlobalVariable.

This patch add the check for the ConstantArray as well as a debug message that
outputs the contents of MustKeepGlobalVariables.

Reviewed By: lei, amyk, scui

Differential Revision: https://reviews.llvm.org/D128287