[Sema] Avoids an assertion failure when an invalid conversion declaration is used

Summary:
When using a user-defined conversion function template with a deduced return type the compiler gives a set of warnings:
```
bug.cc:252:44: error: cannot specify any part of a return type in the declaration of a conversion function; use an alias template to declare a conversion to 'auto (Ts &&...) const'
  template <typename... Ts> operator auto()(Ts &&... xs) const;
                                           ^~~~~~~~~~~~~~~~~~~
bug.cc:252:29: error: conversion function cannot convert to a function type
  template <typename... Ts> operator auto()(Ts &&... xs) const;
                            ^
error: pointer to function type cannot have 'const' qualifier
```
after which it triggers an assertion failure. It seems the last error is incorrect and doesn't have any location information. This patch stops the compilation after the second warning.

Fixes bug 31422.

Patch by Mark de Wever!

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: bbannier, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64820

llvm-svn: 373862