Improve detection of trailing return types.

Trailing return types can only occur in declaration contexts.

Before:
  void f() { auto a = b -> c(); }

After:
  void f() { auto a = b->c(); }

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186087 91177308-0d34-0410-b5e6-96231b3b80d8