[analyzer] Fix StdLibraryFunctionsChecker crash on macOS

Summary:
EOF macro token coming from a PCH file on macOS while marked as literal,
doesn't contain any literal data.  This causes crash on every project
using PCHs.

This commit doesn't resolve the problem with PCH (maybe it was
designed like this for a purpose) or with `tryExpandAsInteger`, but
rather simply shoots off a crash itself.

Differential Revision: https://reviews.llvm.org/D81916