XFAIL one sanitizer symbolizer test for FreeBSD

Summary:
Due to a QoI issuse in FreeBSD's libcxxrt-based demangler, one sanitizer
symbolizer test consistently appears to fail:

    Value of: DemangleSwiftAndCXX("foo")
      Actual: "float"
    Expected: "foo"

This is because libcxxrt's __cxa_demangle() incorrectly demangles the "foo"
identifier to "float".  It should return an error instead.

For now, XFAIL this particular test for FreeBSD, until we can fix libcxxrt
properly (which might take some time to coordinate with upstream).

Reviewers: rnk, zaks.anna, emaste

Subscribers: emaste, llvm-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D23001

llvm-svn: 277297