fold the special-value symbol too under case insensitivity

[Ramya-9353]

Repro: build a converter with ALLOW_CASE_INSENSITIVITY and the EcmaScript symbols infinity_symbol_ = "Infinity", nan_symbol_ = "NaN". StringToDouble("Infinity"), StringToDouble("infinity") and StringToDouble("INFINITY") all return junk_string_value with processed_characters_count == 0; the same holds for every casing of NaN. Lower-case symbols ("infinity"/"nan") are matched fine, which is why the existing case-insensitive test never caught it.

Cause: the fold is applied to the input character only. ConsumeFirstCharacter compares ToLower(input) == symbol_first and ConsumeSubStringImpl compares converter(input) != symbol_byte, both leaving the symbol side un-folded. Any symbol byte that is an upper-case letter therefore never equals the lower-cased input, so no input matches, not even the symbol's own spelling.

Fix: fold both sides. ToLower(first) in ConsumeFirstCharacter, and converter(CodeUnit(*substring)) in ConsumeSubStringImpl and its assert. The case-sensitive path passes Pass (identity), so it keeps rejecting a spelling that differs only in case. Regression test added beside the existing case-insensitive cases, using upper-case symbols across both the char and uc16 overloads.

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[floitsch]

Thanks.