[DebugInfo, CallSites, test] Fix use of undef FileCheck var

Clang test CodeGen/debug-info-extern-call.c tries to check for the
absence of a sequence of instructions with several CHECK-NOT with one of
those directives using a variable defined in another. However CHECK-NOT
are checked independently so that is using a variable defined in a
pattern that should not occur in the input.

This commit removes the CHECK-NOT for the retained line attribute
definition since the CHECK-NOT on the compile unit will already check
that there is no retained lines.

Reviewed By: djtodoro

Differential Revision: https://reviews.llvm.org/D99830