Prevent getTypeAtLocation crash on type-only import clause

[lsh4711]

Fixes #64467

Context

I'm building syscript, a compiler that compiles TypeScript syntax to C for systems
programming. It uses the TypeScript 7 API to type-check the program and then walks
every node, requesting its type in a batch with getTypeAtLocation. That walk crashed
the API server on the first file that used import type.

Problem

checker.GetTypeAtLocation panics with a nil pointer dereference when called on the
ImportClause of a type-only import without a default binding:

// types.ts
export type U = number;
// main.ts
import type { U } from "./types";
import type * as types from "./types";

panic: runtime error: invalid memory address or nil pointer dereference
checker.(*Checker).tryGetDeclaredTypeOfSymbol
checker.(*Checker).getDeclaredTypeOfSymbol
checker.(*Checker).getTypeOfNode
checker.(*Checker).GetTypeAtLocation

It reproduces with the latest nightly (typescript@7.1.0-dev.20260926.1).

Cause

ast.IsTypeDeclaration returns true for a type-only ImportClause, so getTypeOfNode
takes the type-declaration branch and passes the result of getSymbolOfDeclaration
straight to getDeclaredTypeOfSymbol. An import clause only has a symbol when it has
a default binding (import type X from "..."), so for import type { U } and
import type * as ns the symbol is nil.

The missing check looks historical rather than intentional. When this branch was
written, isTypeDeclaration only covered type parameters, classes, interfaces, type
aliases and enums, which always have a symbol. #35200 (type-only imports and exports)
added ImportClause, ImportSpecifier and ExportSpecifier; the specifiers always
have a symbol, but a clause without a default binding does not. The same unguarded
branch is still in Strada's getTypeOfNode, and this code was ported from it.

Fix

Return errorType when the declaration has no symbol, matching the neighboring
IsTypeDeclarationName and IsDeclaration branches, which already guard against a nil
symbol.

This also makes type-only clauses consistent with regular ones: a regular
import { U } from "./types" clause is not a type declaration, so it already reaches
the IsDeclaration branch and gets errorType there.

Tests

Added TestGetTypeAtLocationOfTypeOnlyImportClause in internal/checker. It requests
the type of a named and a namespace type-only import clause and checks that both match
the type of an equivalent regular import clause. It panics without the fix and passes
with it.

Ran the pre-submission checklist from CONTRIBUTING.md. Everything passes except
internal/vfs/osvfs TestOS/Realpath, which also fails on main in my environment
because my home directory is a symlink (/home → /var/home, Fedora Atomic).

---

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[lsh4711]

@microsoft-github-policy-service agree

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