Improve -Z crate-attr diagnostics

[jyn514]

• Show the #![ ... ] in the span (to make it clear that it should not
  be included in the CLI argument)
• Show more detailed errors when the crate has valid token trees but
  invalid syntax.
  Previously, crate-attr=feature(foo),feature(bar) would just say
  "invalid crate attribute" and point at the comma. Now, it explicitly
  says that the comma was unexpected, which is useful when using
  --error-format=short. It also fixes the column to show the correct
  span.
• Recover from parse errors. Previously we would abort immediately on
  syntax errors; now we go on to try and type-check the rest of the
  crate.

The new diagnostic code also happens to be slightly shorter.

r? diagnostics

[petrochenkov]

Move crate-attr parsing from rustc_builtin_macros to
rustc_parse. This is not a built-in macro.

All kind of code injected by rustc consistently lives in rustc_builtin_macros - prelude injection, test harness, etc (search for ::inject(), I suggest keeping it in the current location.
(The logic is that attribute/prelude/etc injection is not used to parse anything, so it's not in rustc_parse.)

[jyn514]

All kind of code injected by rustc consistently lives in rustc_builtin_macros - prelude injection, test harness, etc (search for ::inject(), I suggest keeping it in the current location.
(The logic is that attribute/prelude/etc injection is not used to parse anything, so it's not in rustc_parse.)

ok, done. fortunately parse_attribute was public (i guess another motivation of not having this in rustc_parse is so that we can't use parser internals).