Even though the content of the minidump does no 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 it's job.
Bug: https://bugs.llvm.org/show_bug.cgi?id=34510
Differential Revision: https://reviews.llvm.org/D37527