[FileCheck] Fix comment in parseNumericVariableUse

Summary:
Comment explaining the interaction between parsing of numeric variable
definition and uses in parseNumericVariableUse is stale since it
suggests both use and definition parsing is done in the same function.
This was the case in a previous version of the patch committed as
71d3f227 but is no longer the case. This
patch updates the comment accordingly.

Reviewers: jhenderson, chandlerc, jdenny, probinson, grimar, arichardson, rnk

Subscribers: JonChesterfield, rogfer01, hfinkel, kristina, rnk, tra, arichardson, grimar, dblaikie, probinson, llvm-commits, hiraditya

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64227

llvm-svn: 365192