[LiveDebugVariables] Use lexical scope to trim debug value live intervals

The debug value live intervals computed by Live Debug Variables may extend
beyond the range of the debug location's lexical scope. In this case,
splitting of an interval can result in an interval outside of the scope being
created, causing extra unnecessary DBG_VALUEs to be emitted. To prevent this,
trim the intervals to the lexical scope.

This resolves PR33730.

Reviewers: aprantl

Differential Revision: https://reviews.llvm.org/D35953

llvm-svn: 309933