C++: Don't strip specifiers in Node.getType

[MathiasVP]

We were calling getUnspecifiedType instead of getUnderlyingType in a bunch of places when computing the type of a dataflow node, and since getUnspecifiedType strips away specifiers such as const and volatile we could get into a situation where a node's getType returned int while the same node's asExpr().getType() returned const int.

The fix is to use getUnderlyingType instead of getUnspecifiedType since getUnderlyingType only resolves typedefs without stripping specifiers.

Finally, I also fixed a bug where some nodes were not correctly specified as being dataflow nodes wrapping a glvalue instruction or operand. This also fixed some inconsistency errors.

There still are some type-related bugs, but they seem a bit more difficult to fix. I'll write up an issue for the remaining ones.

Commit-by-commit review recommended.

[MathiasVP]

DCA reports one additional new result that looks like:

const char* patterns[] = { "%s", "file=%s", "\"%s\"", "file=\"%s\"" };
char buf[1 * K];
for (size_t i = 0; i < ARRAY_SIZE(patterns); i++) {
  int ret = jio_snprintf(buf, sizeof(buf), patterns[i], TestLogFileName);
  /* ... */
}

where we claim that patterns[i] isn't a constant format string. I think that's a query FP that I expect to be fixed by #15516. So I don't think that should block this PR.