Add -Wcast-calling-convention to warn when casting away calling conventions

Summary:
This only warns on casts of the address of a function defined in the
current TU. In this case, the fix is likely to be local and the warning
useful.

Here are some things we could experiment with in the future:
- Fire on declarations as well as definitions
- Limit the warning to non-void function prototypes
- Limit the warning to mismatches of caller and callee cleanup CCs

This warning is currently off by default while we study its usefulness.

Reviewers: thakis, rtrieu

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D17348

llvm-svn: 269116