ARROW-7250: [C++] Define constexpr symbols explicitly in StringToFloatConverter::Impl
ARROW-7250: [C++] Define constexpr symbols explicitly in StringToFloatConverter::Impl

Kudos to @pitrou for the solution.

Closes #5891 from xhochy/ARROW-7250 and squashes the following commits:

16eb0d0a5 <Uwe L. Korn> ARROW-7250:  Define constexpr symbols explicitly in StringToFloatConverter::Impl

Authored-by: Uwe L. Korn <5f6f0a63dee51498bdd8036770b3bad30ce7a5ac@quantco.com>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

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