[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