--cfg parse failures produces hard to understand errors

[brson]

rustc --cfg "a{" ../src/test/run-pass/hello.rs

produces:

cfgspec:1:3: 1:3 error: this file contains an un-closed delimiter
cfgspec:1 a{
            ^
cfgspec:1:2: 1:3 help: did you mean to close this delimiter?
cfgspec:1 a{
           ^

It's clear that the way this is parsed is by constructing a fake source file and running it through the main parser. This is a confusing and ugly way to report these errors. Instead the errors should be converted to display in a way that makes it clear that what isn't parsing is the cfg provided on the command line. Perhaps like

error: invalid cfg spec passed on the command line: `a{`

Translating all possible parse errors to something that makes sense in a non-file context may be tricky, so this example does lose information about what the error was.

cc @nrc since you've done some parser refactoring recently.

[birkenfeld]

Current compiler (after fix for #31497) says (last line is new):

cfgspec:1:3: 1:3 error: this file contains an un-closed delimiter
cfgspec:1 a{
            ^
cfgspec:1:2: 1:3 help: did you mean to close this delimiter?
cfgspec:1 a{
           ^
error: invalid --cfg argument: a{