[FileCheck] Simplify numeric variable interface

Summary:
This patch simplifies 2 aspects in the FileCheckNumericVariable code.

First, setValue() method is turned into a void function since being
called only on undefined variable is an invariant and is now asserted
rather than returned. This remove the assert from the callers.

Second, clearValue() method is also turned into a void function since
the only caller does not check its return value since it may be trying
to clear the value of variable that is already cleared without this
being noteworthy.

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/D64231

> llvm-svn: 365249

llvm-svn: 365625