[Frontend] Delete -print-decl-contexts

Summary: Its job is covered by -ast-dump. The option is rarely used and lacks many AST nodes which will lead to llvm_unreachable() crash.

Reviewers: rsmith, arphaman

Reviewed By: rsmith

Subscribers: jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D52529

llvm-svn: 343660