[IR] Allow destruction of symbol table entries regardless of DiscardValueNames

[IR] Allow destruction of symbol table entries regardless of DiscardValueNames

Value::setNameImpl() is used both to set and reset name of the value.
In destructor of Function all arguments get reset their names
(see Function::clearArguments()). If the arguments had their names set (e.g.
when the function was created with LLVMContex::DiscardValueNames == true)
then their ValueName entries referred by the function's symbol table must be
destructed. They are not destructed if LLVMContex::DiscardValueNames is set to
false because of the fast path in Value::setNameImpl(). See the new test cases
that demonstrate the problem. Without the fix they both crash in the function's
destructor.

In Value::setNameImpl() this patch narrows down the fast path return for
DiscardValueNames == true to allow destruction of ValueName entries if any.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D143487