Bug 24894 – [Lexical] Undocumented HexFloat syntax

This code compiles: ```d void main(){ float x = 0xp1; } ``` However, the grammar in the spec doesn't list any way to write a HexFloat without leading digits: ``` HexFloat: HexPrefix HexDigitsNoSingleUS . HexDigitsNoStartingUS HexExponent HexPrefix . HexDigitsNoStartingUS HexExponent HexPrefix HexDigitsNoSingleUS HexExponent ``` So we need to either add ``HexPrefix HexExponent`` to the grammar, or deprecate``0xp1`` in case it wasn't meant to work in the first place.

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