[clang]: Add DeclContext::dumpAsDecl().

This change enables a declaration to be conveniently displayed within
a debugger when only a pointer to its DeclContext is available. For example,
in gdb:
  (gdb) p Ctx
  $1 = (const clang::DeclContext *) 0x14c1a580
  (gdb) p Ctx->dumpAsDecl()
  ClassTemplateSpecializationDecl 0x14c1a540 <t.cpp:1:1, line:7:1> line:2:8 struct ct
  `-TemplateArgument type 'int'
    `-BuiltinType 0x14bac420 'int'
  $2 = void

In the event that the pointed to DeclContext is invalid (that it has an
invalid DeclKind as a result of a dangling pointer, memory corruption, etc...)
it is not possible to dump its associated declaration. In this case, the
DeclContext will be reported as invalid. For example, in gdb:
  (gdb) p Ctx->dumpAsDecl()
  DeclContext 0x14c1a580 <unrecognized Decl kind 127>
  $3 = void