Use fileOffset in generateIdWithLoc

[dkorpel]

Remove the GC string concatenation, and use loc.fileOffset instead, which is more robust since it doesn't get affected by #line directives (which a preprocessed ImportC file tends to have a lot of). Unfortunately many tests still expose the internal name, but I made the ones which can't be AUTO_UPDATE'd less reliant on hard-coded line/column numbers.

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Testing this PR locally

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[dkorpel]

Still todo: deal with mixins creating duplicate locations

void main()
{
    auto z0 = mixin("(() => 3)()");
    auto z1 = mixin("(() => 4)()");
    assert(z0 == 3);
    assert(z1 == 4);
}

And see what to do about buildkite failures from projects testing for hard-coded unittest_LXX_CXX strings.