[FileCheck] Make NumericVariable ctor explicit

[thopre]

Summary

Make FileCheckNumericVariable constructor explicit to avoid implicit
conversions from StringRef.

[jhenderson]

LGTM.

llvm/include/llvm/Support/FileCheck.h

private:
   /// Line number where this variable is defined, or None if defined before
   /// input is parsed. Used to determine whether a variable is defined on the
   /// same line as a given use.
   Optional<size_t> DefLineNumber;
 
 public:
   /// Constructor for a variable \p Name defined at line \p DefLineNumber or
   /// defined before input is parsed if \p DefLineNumber is None.
-  FileCheckNumericVariable(StringRef Name,
+  explicit FileCheckNumericVariable(StringRef Name,
                                     Optional<size_t> DefLineNumber = None)
       : Name(Name), DefLineNumber(DefLineNumber) {
   }
 
   /// \returns name of this numeric variable.
   StringRef getName() const { return Name; }
 
   /// \returns this variable's value.
   Optional<uint64_t> getValue() const { return Value; }