Adopt a more principled approach to invalid declarations:

  - If a declaration is an invalid redeclaration of an existing name,
    complain about the invalid redeclaration then avoid adding it to
    the AST (we can still parse the definition or initializer, if any).
  - If the declaration is invalid but there is no prior declaration
    with that name, introduce the invalid declaration into the AST
    (for later error recovery).
  - If the declaration is an invalid redeclaration of a builtin that
    starts with __builtin_, we produce an error and drop the
    redeclaration. If it is an invalid redeclaration of a library
    builtin (e.g., malloc, printf), warn (don't error!) and drop the
    redeclaration.

If a user attempts to define a builtin, produce an error and (if it's
a library builtin like malloc) suggest -ffreestanding.

This addresses <rdar://problem/6097585> and PR2892. However, PR3588 is
still going to cause some problems when builtins are redeclared
without a prototype.

llvm-svn: 64639