Revert r321697 "[libclang] Support querying whether a declaration is invalid" and follow-ups.

This broke test/Index/opencl-types.cl on several buildbots:
http://lab.llvm.org:8011/builders/clang-cmake-aarch64-lld/builds/3294
http://lab.llvm.org:8011/builders/clang-ppc64be-linux-multistage/builds/6498
http://lab.llvm.org:8011/builders/clang-ppc64le-linux-multistage/builds/5239

> [libclang] Support querying whether a declaration is invalid
>
> This is useful for e.g. highlighting purposes in an IDE.
>
> Patch by Nikolai Kosjar.
>
> Differential Revision: https://reviews.llvm.org/D40072

Also reverting follow-ups that otherwise caused conflicts for the
revert:

r321700 "Fix line endings."
r321701 "Fix more line endings."

r321698 "[libclang] Fix cursors for functions with trailing return type"
> For the function declaration
>
> auto foo5(Foo) -> Foo;
> the parameter tokens were mapped to cursors representing the
> FunctionDecl:
>
> Keyword: "auto" [1:1 - 1:5] FunctionDecl=test5:1:6
> Identifier: "test5" [1:6 - 1:11] FunctionDecl=test5:1:6
> Punctuation: "(" [1:11 - 1:12] FunctionDecl=test5:1:6
> Identifier: "X" [1:12 - 1:13] FunctionDecl=test5:1:6 // Ops, not a TypeRef
> Punctuation: ")" [1:13 - 1:14] FunctionDecl=test5:1:6
> Punctuation: "->" [1:15 - 1:17] FunctionDecl=test5:1:6
> Identifier: "X" [1:18 - 1:19] TypeRef=struct X:7:8
> Punctuation: ";" [1:19 - 1:20]
>
> Fix this by ensuring that the trailing return type is not visited as
> first.
>
> Patch by Nikolai Kosjar.
>
> Differential Revision: https://reviews.llvm.org/D40561

llvm-svn: 321708