Lower the minimum number of affected pointers for -warn-root-cause from 2 to 1

[mattmccutchen-cci]

Currently, 3c -warn-root-cause shows a root cause only if its location is in one of the main source files or it affects at least 2 relevant pointers:

checkedc-clang/clang/lib/3C/RewriteUtils.cpp

Line 572 in e1897b5

if (WarnAllRootCause || SM.isInMainFile(SL) || PtrCount > 1) {

This is unhelpful for the tutorial, in which after the first 3C pass, only one wild pointer is left and the user has no good way to find it because its root cause has only 1 affected pointer. In -warn-all-root-cause, the important root cause is buried in a pile of other root causes for unwritable code with 0 affected relevant pointers.

We should probably change -warn-root-cause to show all root causes that affect at least 1 relevant pointer. It looks like the original decision to set the threshold at 2 was made here; that may have made sense at the time (possibly because 3C handled unwritable code differently at the time?), but I don't see any justification for it now.