If a function decl cannot be merged, mark it as invalid.

Clang currently crashes on

    class C {
      C() = default;
      C() = delete;
    };

My cunning plan for fixing this was to change the `if (!FnD)` in
Parser::ParseCXXInlineMethodDef() to `if (!FnD || FnD->isInvalidDecl)` – but
alas, the second constructor decl wasn't marked as invalid.  This lets
Sema::MergeFunctionDecl() return true on function redeclarations, which leads
to them being marked invalid.

This also improves error messages when functions are redeclared.

llvm-svn: 226365