[analyzer] Shitfing a constant value by its bit width is undefined.

Citation: C++11 [expr.shift]p1 (and the equivalent text in C11).

This fixes PR18073, but the right thing to do (as noted in the FIXME) is to
have a real checker for too-large shifts.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199405 91177308-0d34-0410-b5e6-96231b3b80d8