Detect an incompatible VLA pointer assignment

For pointer assignments of VLA types, Clang currently detects when array
dimensions _lower_ than a variable dimension differ, and reports a warning.
However it does not do the same when the _higher_ dimensions differ, a
case that GCC does catch.

These two pointer types

    int (*foo)[1][bar][3];
    int (*baz)[1][2][3];

are compatible with each another, and the program is well formed if
bar == 2, a matter that is the programmers problem. However the following:

    int (*qux)[2][2][3];

would not be compatible with either, because the upper dimension differs
in size. Clang reports baz is incompatible with qux, but not that foo is
incompatible with qux because it doesn't check those higher dimensions.

Fix this by comparing array sizes on higher dimensions: if both are
constants but unequal then report incompatibility; if either dimension is
variable then we can't know either way.

Differential Revision: https://reviews.llvm.org/D47628

llvm-svn: 333989