[FileCheck] Added --enable-var-scope option to enable scope for regex variables.

If `--enable-var-scope` is in effect, variables with names that
start with `$` are considered to be global. All other variables are
local. All local variables get undefined at the beginning of each
CHECK-LABEL block. Global variables are not affected by CHECK-LABEL.
This makes it easier to ensure that individual tests are not affected
by variables set in preceding tests.

Differential Revision: https://reviews.llvm.org/D30749

llvm-svn: 297396