[clang][dataflow] Create `Value`s for integer literals.

This patch includes a test that fails without the fix.

I discovered that we weren't creating `Value`s for integer literals when, in a
different patch, I tried to overwrite the value of a struct field with a literal
for the purposes of a test and was surprised to find that the struct compared
the same before and after the assignment.

This functionality therefore seems useful at least for tests, but is probably
also useful for actual analysis of code.

Reviewed By: ymandel, xazax.hun, gribozavr2

Differential Revision: https://reviews.llvm.org/D152813