[analyzer] Only attempt to get the value of locations of known type

Fixes https://bugs.llvm.org/show_bug.cgi?id=36474

In general, getSVal API should be changed so that it does not crash on
some non-obvious conditions.
It should either be updated to require a type, or to return Optional<SVal>.

Differential Revision: https://reviews.llvm.org/D43801

llvm-svn: 326233