[FileCheck] Factor some parsing checks out

Summary:
Both callers of parseNumericVariableDefinition() perform the same extra
check that no character is found after the variable name. This patch
factors out this check into parseNumericVariableDefinition().

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/D64226

llvm-svn: 365191