Debugger: Handle constant member offsets properly.

For DWARF4, gcc now sometimes emits member locations as constant
offsets from their parent rather than as location expressions.
We weren't handling this case properly, rather we were always
assuming that a constant offset implied we were dealing with a
bit field.

Fixes struct/class members not always having correct values on
version 4.
Debugger: Fix X86 handling of larger return values.

For return values too large to fit into EAX:EDX, the address at which
the value is located is stored in EAX. We were incorrectly assuming
they were at the current stack pointer, which isn't necessarily the
case depending on what optimization/code generation options were
used, leading to us sometimes showing the wrong values for such case.
The same issue showed up with debug binaries generated by Clang.