Rework the warning for 'memset(p, 0, sizeof(p))' where 'p' is a pointer

and the programmer intended to write 'sizeof(*p)'. There are several
elements to the new version:

1) The actual expressions are compared in order to more accurately flag
   the case where the pattern that works for an array has been used, or
   a '*' has been omitted.
2) Only do a loose type-based check for record types. This prevents us
   from warning when we happen to be copying around chunks of data the
   size of a pointer and the pointer types for the sizeof and
   source/dest match.
3) Move all the diagnostics behind the runtime diagnostic filter. Not
   sure this is really important for this particular diagnostic, but
   almost everything else in SemaChecking.cpp does so.
4) Make the wording of the diagnostic more precise and informative. At
   least to my eyes.
5) Provide highlighting for the two expressions which had the unexpected
   similarity.
6) Place this diagnostic under a flag: -Wsizeof-pointer-memaccess

This uses the Stmt::Profile system for computing #1. Because of the
potential cost, this is guarded by the warning flag. I'd be interested
in feedback on how bad this is in practice; I would expect it to be
quite cheap in practice. Ideas for a cheaper / better way to do this are
also welcome.

The diagnostic wording could likely use some further wordsmithing.
Suggestions welcome here. The goals I had were to: clarify that its the
interaction of 'memset' and 'sizeof' and give more reasonable
suggestions for a resolution.

An open question is whether these diagnostics should have the note
attached for silencing by casting the dest/source pointer to void*.

llvm-svn: 133155

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