Sign the positive infinity token so flatc 1.12 accepts it

[rascani]

Summary:
The non-finite float rewrite added in the parent diff spells positive
infinity as a bare inf, which fails to lower under the flatc shipped
in fbsource (1.12.0). flatc parses a VkValue union member's value
before it knows the member's type, so a token that starts with a letter
(inf, nan) is rejected as an unknown value:

schema.json:1: 128: error: cannot parse value starting with: inf

A signed token takes the numeric path and parses, so this emits +inf
for positive infinity (-inf was already correct). flatc writes the
bare inf back on decompile, which the existing reverse rewrite already
maps to json's Infinity, so only the forward direction needed the fix.

This unbreaks test_serialize_deserialize_non_finite_scalars and
test_serialize_deserialize_non_finite_floats_in_list.

Reviewed By: rascani

Differential Revision: D119675660

[pytorch-bot]

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✅ You can merge normally! (4 Unrelated Failures)

As of commit 90e0267 with merge base 1abaa87 ():

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
  backends/native/test/test_passes.py::ReplaceCopyWithAliasPassTest::test_dynamic_view_converts_to_alias
• pull / unittest / macos / macos-job (gh) (trunk failure)
  backends/native/test/test_passes.py::ReplaceCopyWithAliasPassTest::test_dynamic_view_converts_to_alias
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  backends/native/test/test_passes.py::ReplaceCopyWithAliasPassTest::test_dynamic_view_converts_to_alias
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  backends/native/test/test_passes.py::ReplaceCopyWithAliasPassTest::test_dynamic_view_converts_to_alias

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[meta-codesync]

@rascani has exported this pull request. If you are a Meta employee, you can view the originating Diff in D119675660.

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