[FileCheck] Make NumericVariable ctor explicit

Summary:
Make FileCheckNumericVariable constructor explicit to avoid implicit
conversions from StringRef.

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

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66640

llvm-svn: 370664