bug 1685178: fix signature generation for unknown in dll frames
bug 1685178: fix signature generation for unknown in dll frames

We changed dump_syms to yield a better symbol for cases where we have a
dll without symbols. The better symbol is structured like:

<unknown in foo.dll>

The < > triggers the collapsing types in the CSignatureTool
normalization code. This adds an exception so that these frames don't
get converted to <T>.

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