Don't treat a declaration as a type argument position (#25046)

src/services/utilities.ts

@@ -948,7 +948,7 @@ namespace ts {
                     token = findPrecedingToken(token.getFullStart(), sourceFile);
                     if (!token || !isIdentifier(token)) return undefined;
                     if (!remainingLessThanTokens) {
-                        return { called: token, nTypeArguments };
+                        return isDeclarationName(token) ? undefined : { called: token, nTypeArguments };
                     }
                     remainingLessThanTokens--;
                     break;

tests/cases/fourslash/signatureHelpOnDeclaration.ts

@@ -0,0 +1,6 @@
+/// <reference path='fourslash.ts' />
+
+////function f</**/
+////x
+
+verify.noSignatureHelp("");