compiler: rationalize external symbol names

compiler: rationalize external symbol names

    Encode all external symbol names using only ASCII alphanumeric
    characters, underscore, and dot.  Use a scheme that can be reliably
    demangled to a somewhat readable version as described in the long
    comment in names.cc.

    A minor cleanup discovered during this was that we were treating
    function types as different if one had a NULL parameters_ field and
    another has a non-NULL parameters_ field that has no parameters.  This
    worked because we mangled them slightly differently.  We now mangle
    them the same, so we treat them as equal, as we should anyhow.

    Reviewed-on: https://go-review.googlesource.com/89555

	* go.go-torture/execute/names-1.go: New test.

From-SVN: r257033