[AST] Override getLocFromSource for ParamDecl

Ensure we can still produce a source location even if there's no name
loc, which is the case for e.g enum associated types.

Author: hamishknight

include/swift/AST/Decl.h

@@ -7101,6 +7101,14 @@ class ParamDecl : public VarDecl {
       Identifier parameterName, Expr *defaultValue,
       DefaultArgumentInitializer *defaultValueInitContext, DeclContext *dc);
 
+  SourceLoc getLocFromSource() const {
+    // If we have a name loc, use it, otherwise fallback to the start loc for
+    // e.g enum elements without parameter names.
+    if (auto nameLoc = getNameLoc())
+      return nameLoc;
+    return getStartLoc();
+  }
+
   /// Retrieve the argument (API) name for this function parameter.
   Identifier getArgumentName() const {
     return ArgumentNameAndFlags.getPointer();

validation-test/compiler_crashers_2/1b94fce977ad935d.swift renamed to validation-test/compiler_crashers_2_fixed/1b94fce977ad935d.swift

@@ -1,3 +1,3 @@
 // {"kind":"typecheck","signature":"printDifferentiableAttrArguments(swift::DifferentiableAttr const*, swift::ASTPrinter&, swift::PrintOptions const&, swift::Decl const*, bool)","signatureAssert":"Assertion failed: (original && \"Must resolve original declaration\"), function printDifferentiableAttrArguments"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 func a(@differentiable _ =