Concurrency: Use std::bit_cast over ignoring -Wcast-function-type-mismatch

[tshortli]

Building on #78985, use std::bit_cast consistently wherever Clang had been emitting -Wcast-function-type-mismatch warnings, instead of ignoring the diagnostic.

[compnerd]

Nice! Thanks for cleaning up the other instances.

[tshortli]

@swift-ci please smoke test

[tshortli]

Looks like Windows CI is unhappy:

In file included from C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\stdlib\public\Concurrency\Task.cpp:38:
C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\include\swift/Runtime/STLCompatibility.h(30,1): error: definition with same mangled name '??$bit_cast@P6WXPEAVAsyncContext@swift@@@ZP6WX0PEAX@Z@__swift@std@@YAP6WXPEAVAsyncContext@swift@@@ZAEBQ6WX0PEAX@Z@Z' as another definition
   30 | bit_cast(const Source &src) noexcept {
      | ^
C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\include\swift/Runtime/STLCompatibility.h(30,1): note: previous definition is here
1 error generated.

[compnerd]

interesting, it is claiming that there is a duplicate definition.

[compnerd]

I think that this might be a bug in clang? There are a few reports of this being caused with concepts.