Concurrency: remove workaround for silencing UB

The newer clang properly identifies UB on invalid pointer casts. This
was previously being silenced by suppressing the warnings. Adjust the
code to use `std::bit_cast` (or the shim implementation) to avoid the
UB in this code.

test/embedded/dependencies-concurrency-custom-executor.swift

@@ -18,6 +18,7 @@
 // DEP: _exit
 // DEP: _free
 // DEP: _malloc
+// DEP: _memcpy
 // DEP: _memmove
 // DEP: _memset
 // DEP: _memset_s