Debug Info: code clean up.

Remove unnecessary creation of LexicalScope in collectDeadVariables.
The created LexicialScope was only used to get isAbstractScope, which
should be false from the creation:
"new LexicalScope(NULL, DIDescriptor(SP), NULL, false);".

We can also remove a DenseMap that holds the created LexicalScopes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193196 91177308-0d34-0410-b5e6-96231b3b80d8