[Lexer] Add a few test cases for hex number literal

[rintaro]

What's in this pull request?

Follow up on #3124.
Added several test cases.

[rintaro]

@swift-ci Please test and merge

[jckarter]

There's also the case of 0x1.p or 0x1.p2. The first one isn't a valid float literal, but the second one is a toss-up—it'd be accepted as a float literal in C, at least.

[jckarter]

I guess we already diverge from C by requiring a digit after the dot in decimal literals (IOW 1. isn't accepted) so it'd be reasonable to require the same of hex float literals.

[rintaro]

Current published specification requires "." ­hexadecimal-digit ....
So as implementation: https://github.com/apple/swift/blob/a5eed38/lib/Parse/Lexer.cpp#L872-L877

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[jckarter]

Makes sense, thanks. Got time to add those as test cases then?

[rintaro]

I'd be happy to do that, but I'm very tired today.
I will do that in tomorrow. (in Japan).

[jckarter]

No problem, thanks for fixing this!