[DebugInfo] Use DbgVariableIntrinsic as the base class of variables.

After refactoring DbgInfoIntrinsic class hierarchy, we use
DbgVariableIntrinsic as the base class of variable debug info.

In resolveTopLevelMetadata() in CGVTables.cpp, we only care about
dbg.value, so we try to cast the instructions to DbgVariableIntrinsic
before resolving variables.

Differential Revision: https://reviews.llvm.org/D50226

llvm-svn: 338985