[MIR] Support +inf and -inf literals

[Inconnu08]

Support parsing +inf and -inf as floating-point literals in MIR.

The MIR lexer previously tokenized the leading sign separately, causing
G_FCONSTANT float +inf and G_FCONSTANT float -inf to fail with
"expected a floating point literal".

Recognize signed infinity spellings as FloatingPointLiteral tokens so
they can be handled by the existing floating-point constant parsing path.

Add a regression test covering both positive and negative infinity.

Fixes #221040

Assisted-by: ChatGPT (rubber ducking)

[github-actions]

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[Inconnu08]

I've read the LLVM AI Tool Use Policy and the linked contribution policies. AI assistance has been disclosed in the PR description, and I've reviewed, tested, and understand the submitted changes.