PR21969: Improve diagnostics for a conversion function that has any pieces of a
declared return type (including a trailing-return-type in C++14).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224561 91177308-0d34-0410-b5e6-96231b3b80d8
Fix layering violation, Lex shouldn't know about Decl

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224562 91177308-0d34-0410-b5e6-96231b3b80d8
[Sanitizer] Get rid of parseArgument helper function. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224564 91177308-0d34-0410-b5e6-96231b3b80d8