[analyzer] Warn when passing pointers to const but uninitialized memory.

Passing a pointer to an uninitialized memory buffer is normally okay,
but if the function is declared to take a pointer-to-const then it's
very unlikely it will be modifying the buffer. In this case the analyzer
should warn that there will likely be a read of uninitialized memory.

This doesn't check all elements of an array, only the first one.
It also doesn't yet check Objective-C methods, only C functions and
C++ methods.

This is controlled by a new check: alpha.core.CallAndMessageUnInitRefArg.

Patch by Per Viberg!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203822 91177308-0d34-0410-b5e6-96231b3b80d8