[remark][diagnostics] [codegen] Fix PR44896

This patch fixes PR44896. For IR input files, option fdiscard-value-names
should be ignored as we need named values in loadModule().
Commit 60d39479 sets this option after loadModule() where valued names
already created. This creates an inconsistent state in setNameImpl()
that leads to a seg fault.
This patch forces fdiscard-value-names to be false for IR input files.

This patch also emits a warning of "ignoring -fdiscard-value-names" if
option fdiscard-value-names is explictly enabled in the commandline for
IR input files.

Differential Revision: https://reviews.llvm.org/D74878