[lldb] Fix handling of cfi_restore in the unwinder

Currently, lldb's unwinder ignores cfi_restore opcodes for registers
that are not set in the first row of the unwinding info. This prevents
unwinding of failed assertion in Chrome/v8 (https://github.com/v8/v8).
The attached test is an x64 copy of v8's function that failed to unwind
correctly (V8_Fatal).

This patch changes handling of cfi_restore to reset the location if
the first unwind table row does not map the restored register.

Differential Revision: https://reviews.llvm.org/D153043