[FileCheck] Store line numbers as optional values

Summary:
Processing of command-line definition of variable and logic around
implicit not directives both reuse parsing code that expects a line
number to be defined. So far, a special line number of 0 was used for
those users of the parsing code where a line number does not make sense.
This commit instead represents line numbers as Optional values so that
they can be None for those cases.

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/D64639

llvm-svn: 366109