Improve expression evaluator, (non)fatal attribute evaluation, and ORC dogfood

[fosterbrereton]

This PR does a handful of things:

Improved expression evaluation

Some libraries have been found that use new DWARF 5 constructs (DW_OP_stack_value), as well as more complex uses of the DWARF form expression evaluator. This PR improves upon ORC's ability to evaluate DWARF form expressions. This will fix some errors where the attribute is showing as <unhandled>, and may result in an ODRV false positives that read thusly (note the <unhandled> value for the first data_member_location):

warning: ODRV (member:data_member_location); conflict in `NSURL::_baseURL`
    within: libAppAuth.a:
    within: OIDAuthState+IOS.o:
        definition location: /path/to/NSURL.h:22
        name: _baseURL
        type: NSURL*
        data_member_location: <unhandled>
        accessibility: 2 (0x2)

    within: libGTMAppAuth.a:
    within: GTMAppAuthFetcherAuthorization.o:
        definition location: /path/to/NSURL.h:22
        name: _baseURL
        type: NSURL*
        data_member_location: 0 (0x0)
        accessibility: 2 (0x2)

Improved expression evaluation skipping

We have been identifying nonfatal attributes for some time now. This PR includes an optimization where, for some attributes, the value is passed over when we know it is not necessary for detecting for ODRVs.

Dogfood build

The CMake project is modified to add an orc_dogfood target. It is identical to the orc_orc target, except ORC is passed over the dogfood target during link time. This ensures that ORC itself has no ODR violations (at least as far as the tool can detect them.) This addresses a comment made in #20.

[leethomason]

Deep. Looks good and moves the ball down the field.