clang-rename: adjust NamedDeclFindingASTVisitor for RecordDecls

Ensure that Context is always properly initialised in the constructor.  It is
used for querying the LangOpts in VisitTypeLoc.  Prevent a null pointer
dereference in setResult by ensuring that a RecordDecl is being handled.

Patch by Alexander Shaposhnikov!

llvm-svn: 276948