Fix for bug 34510 - Minidump target does not resolve new symbols correctly

Even though the content of the minidump does not change in a debugging session,
frames can't be indiscriminately be cached since modules and symbols can be
explicitly added after the minidump is loaded.

The fix is simple, just let the base Thread::ClearStackFrames() do its job.

submitted by amccarth on behalf of lemo

Bug: https://bugs.llvm.org/show_bug.cgi?id=34510

Differential Revision: https://reviews.llvm.org/D37527

llvm-svn: 312735