[Issue 24894] New: [Lexical] Undocumented HexFloat syntax

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24894

          Issue ID: 24894
           Summary: [Lexical] Undocumented HexFloat syntax
           Product: D
           Version: D2
          Hardware: All
               URL: http://dlang.org/
                OS: All
            Status: NEW
          Severity: normal
          Priority: P3
         Component: dlang.org
          Assignee: nobody at puremagic.com
          Reporter: zxinsworld at gmail.com

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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