Handle StaticAssertDecl in DeclContextPrinter

This commit fixes a crash that occurs when -print-decl-contexts AST consumer
tries to print an unhandled declaration.

rdar://19467234

Differential Revision: https://reviews.llvm.org/D26964

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290887 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 48194d104cad1bd83e71e0e95ccfa7e796bad5b2)
2 files changed