[DebugInfo] Avoid crash from dropped fragments in LiveDebugValues

This patch avoids a crash caused by DW_OP_LLVM_fragments being dropped
from DIExpressions by LiveDebugValues spill-restore code. The appearance
of a previously unseen fragment configuration confuses LDV, as documented
in PR42773, and reproduced by the test function this patch adds (Crashes
on a x86_64 debug build).

To avoid this, on spill restore, we now use fragment information from the
spilt-location-expression.

In addition, when spilling, we now don't spill any DBG_VALUE with a complex
expression, as it can't be safely restored and will definitely lead to an
incorrect variable location. The discussion of this is in D65368.

Differential Revision: https://reviews.llvm.org/D66284

llvm-svn: 369026