[clangd] Fix an assertion failure in Selection.

Summary:
The assertion is triggered when the Decl is null.

Details for the assertion:

F0207 09:55:09.069385   47308 logging.cc:84] assert.h assertion failed at llvm/include/llvm/Support/Casting.h:105 in static bool llvm::isa_impl_cl<clang::TranslationUnitDecl, const clang::      Decl *>::doit(const From *) [To = clang::TranslationUnitDecl, From = const clang::Decl *]: Val && "isa<> used on a null pointer"
 15 *** Check failure stack trace: ***
 19     @     0x55615c1f7e06  __assert_fail
 20     @     0x55615a6297d8  clang::clangd::(anonymous namespace)::SelectionVisitor::TraverseDecl()
 21     @     0x55615a62f48d  clang::RecursiveASTVisitor<>::TraverseTemplateTemplateParmDecl()
 22     @     0x55615a62b264  clang::RecursiveASTVisitor<>::TraverseDecl()
 23     @     0x55615a62979c  clang::clangd::(anonymous namespace)::SelectionVisitor::TraverseDecl()
 24     @     0x55615a63060c  clang::RecursiveASTVisitor<>::TraverseClassTemplatePartialSpecializationDecl()
 25     @     0x55615a62ae45  clang::RecursiveASTVisitor<>::TraverseDecl()

Reviewers: sammccall

Subscribers: javed.absar, kristof.beyls, ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57879

llvm-svn: 353421