[LiveDebugValues] Cache LexicalScopes::getMachineBasicBlocks, NFCI

Summary:
Cache the results from getMachineBasicBlocks in LexicalScopes to speed
up UserValueScopes::dominates queries.  This replaces the caching done
in UserValueScopes. Compared to the old caching method, this reduces
memory traffic when a VarLoc is copied (e.g. when a VarLocMap grows),
and enables caching across basic blocks.

When compiling sqlite 3.5.7 (CTMark version), this patch reduces the
number of calls to getMachineBasicBlocks from 10,207 to 1,093. I also
measured a small compile-time reduction (~ 0.1% of total wall time, on
average, on my machine).

As a drive-by, I made the DebugLoc in UserValueScopes a const reference
to cut down on MetadataTracking traffic.

Reviewers: jmorse, Orlando, aprantl, nikic

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80957