constexpr estimateSpaceNeeded for string literals.
constexpr estimateSpaceNeeded for string literals.

Summary:
Partially inspired by WillerZ's investigation into faster compiling
StringPiece work, I thought this would be an easy drop in that short
circuits some compilation logic (or possibly is strictly better, since the
StringPiece conversion method isn't constexpr).

Reviewed By: yfeldblum

Differential Revision: D6059537

fbshipit-source-id: 072f56e58aa47db10b54825cac8a05dc035b295c

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