Doc attribute normalization is incorrectly detected as comment loss

[dtolnay]

Repro against current master (6495024):

$ echo '#[doc = "..."] static S: () = { #[doc = "..."] struct S; };' | CFG_RELEASE_CHANNEL=dev CFG_RELEASE=dev cargo run --bin rustfmt -- --edition=2018 --emit=stdout --config=normalize_doc_attributes=true

///...
static S: () = { #[doc = "..."] struct S; };

error[internal]: not formatted because a comment would be lost
 --> stdin:1
  |
1 | #[doc = "..."] static S: () = { #[doc = "..."] struct S; };
  |
  = note: set `error_on_unformatted = false` to suppress the warning against comments or string literals

warning: rustfmt has failed to format. See previous 1 errors.

You can see the first #[doc = "..."] is correctly normalized to ///... while the second is not, due to the printed internal error.

This seems like a consequence of normalize_doc_attributes=true because the following is formatted with no problem.

$ echo -e '///...\nstatic S: () = { ///...\nstruct S; };' | CFG_RELEASE_CHANNEL=dev CFG_RELEASE=dev cargo run --bin rustfmt -- --edition=2018 --emit=stdout --config=normalize_doc_attributes=true

///...
static S: () = {
    ///...
    struct S;
};