Skip TestFunctionTemplateParameterPack.py, which unexpectedly asserts (PR35920)

This test stresses expression evaluation support for template functions.
Currently the support is rudimentary, and running this test causes assertion
failures in clang. This test cannot be XFAIL'ed because the test harness
treats assertion failures as unexpected events. For now, the test must be
skipped.

llvm-svn: 322340