Auto merge of #71487 - rcoh:71471-shebang, r=petrochenkov

Fix bug in shebang handling

Shebang handling was too agressive in stripping out the first line in cases where it is actually _not_ a shebang, but instead, valid rust (#70528). This is a second attempt at resolving this issue (the first attempt was reverted, for, among other reasons, causing an ICE in certain cases (#71372, #71471).

The behavior is now codified by a number of UI tests, but simply:
For the first line to be a shebang, the following must all be true:
1. The line must start with `#!`
2. The line must contain a non-whitespace character after `#!`
3. The next character in the file, ignoring comments & whitespace must not be `[`

I believe this is a strict superset of what we used to allow, so perhaps a crater run is unnecessary, but probably not a terrible idea.

Fixes #70528

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