[FileCheck] Add function call support to numerical expressions.

This patch extends numerical expressions to allow calls to
predefined functions. These calls can be combined with the
existing numerical operators, which includes nesting calls.

The call syntax is:

  <func>(<args>)

Where <func> is a predefined string literal, currently limited to
one of add, max, min and sub. <arg> is a comma seperated list of
numerical expressions.

Subscribers: arichardson, hiraditya, thopre, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79936