[CLANG] Fix potential null pointer dereference bugs

This patch uses castAs instead of getAs which will assert if the type doesn't match and adds nullptr check if needed.

Also this patch improves the codes and passes I.getData() instead of doing a lookup in dumpVarDefinitionName()
since we're iterating over the same map in LocalVariableMap::dumpContex().

Reviewed By: aaron.ballman, aaronpuchert

Differential Revision: https://reviews.llvm.org/D153033