Add experimental -Wstrlcpy-size warning that looks to see if the size argument...

Add experimental -Wstrlcpy-size warning that looks to see if the size argument for strlcpy/strlcat is the size of the *source*, and not the size of the *destination*.  This warning is off by default (for now).

Warning logic provided by Geoff Keating.

llvm-svn: 137903