Fix/promise task declaration order

[patrickkabwe]

Follow up of #1474

AssertPromiseState.hpp and Promise.hpp include each other, and PromiseTask was declared after the #include "Promise.hpp". That works only when Promise.hpp is entered first true when both are reached through a .cpp, but not when a build system compiles the module's public headers as a clang module and enters AssertPromiseState.hpp first. Promise.hpp's body is then parsed before the enum exists:

Promise.hpp:102:31: error: use of undeclared identifier 'PromiseTask'

Move the declaration up next to the class Promise forward declaration it belongs with, so the header is valid whichever one is entered first.

[vercel]

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