Reinitialize UnwindTable when the SymbolFile changes

Summary:
This is a preparatory step to enable adding of unwind plans by symbol
file plugins.

Although at the surface it seems that currently symbol files have
nothing to do with unwinding, this isn't entirely correct even now. The
mere act of adding a symbol file can have the effect of making more
sections (typically .debug_frame) available to the unwinding machinery,
so that it can have more unwind strategies to choose from.

Up until now, we've had a bug, which went largely unnoticed, where
unwind info in the manually added symbols files (target symbols add) was
being ignored during unwinding. Reinitializing the UnwindTable fixes
that bug too.

Reviewers: clayborg, jasonmolenda, alexshap

Subscribers: jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D58347

llvm-svn: 356361