Fix for Issue #4149, Added undocumented HexFloat syntax to the specification

[Prthmsh7]

Fixes #4149

I've added the missing fourth form to the HexFloat grammar definition,
HexFloat: HexPrefix HexDigitsNoSingleUS . HexDigitsNoStartingUS HexExponent HexPrefix . HexDigitsNoStartingUS HexExponent HexPrefix HexDigitsNoSingleUS HexExponent HexPrefix HexExponent
This change documents the existing behavior of the D compiler, which already accepts the 0xp1 syntax. The value of a hex float literal in this form is interpreted as zero (0) shifted left by the exponent value. Also, rather than deprecating the syntax, I've updated the specification.

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