Add a warning for when an array-to-pointer decay is performed on an array

Add a warning for when an array-to-pointer decay is performed on an array
temporary or an array subobject of a class temporary, and the resulting value
is used to initialize a pointer which outlives the temporary. Such a pointer
is always left dangling after the initialization completes and the array's
lifetime ends.

In order to detect this situation, this change also adds an
LValueClassification of LV_ArrayTemporary for temporaries of array type which
aren't subobjects of class temporaries. These occur in C++11 T{...} and GNU C++
(T){...} expressions, when T is an array type. Previously we treated the former
as a generic prvalue and the latter as a class temporary.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157955 91177308-0d34-0410-b5e6-96231b3b80d8