rustc_driver::main installs a handler that will set this to true if
the compiler has been sent a request to shut down, such as by a Ctrl-C.
This static lives here because it is only read by the interpreter.
Const eval always happens in post analysis mode in order to be able to use the hidden types of
opaque types. This is needed for trivial things like size_of, but also for using associated
types that are not specified in the opaque type. We also use MIR bodies whose opaque types have
already been revealed, so weβd be able to at least partially observe the hidden types anyways.