[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