Signature#declaration can be undefined (#22515)

Author: ajafff

src/compiler/types.ts

@@ -3875,7 +3875,7 @@ namespace ts {
     }
 
     export interface Signature {
-        declaration: SignatureDeclaration;  // Originating declaration
+        declaration?: SignatureDeclaration; // Originating declaration
         typeParameters?: TypeParameter[];   // Type parameters (undefined if non-generic)
         parameters: Symbol[];               // Parameters
         /* @internal */

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -2223,7 +2223,7 @@ declare namespace ts {
         Construct = 1,
     }
     interface Signature {
-        declaration: SignatureDeclaration;
+        declaration?: SignatureDeclaration;
         typeParameters?: TypeParameter[];
         parameters: Symbol[];
     }

tests/baselines/reference/api/typescript.d.ts

@@ -2223,7 +2223,7 @@ declare namespace ts {
         Construct = 1,
     }
     interface Signature {
-        declaration: SignatureDeclaration;
+        declaration?: SignatureDeclaration;
         typeParameters?: TypeParameter[];
         parameters: Symbol[];
     }