swift234

[SILOptimizer] Fix let constant of non-class protocol being mutable

Extend the checks in `LifetimeChecker` in
`SILOptimizer/Mandatory/DefiniteInitialization.cpp`
to catch when the memory object corresponding to a let constant is used
as an inout parameter in a protocol witness method.

Initializing a let constant separate from its declaration requires write
access. Therefore it is treated as `@lvalue TestProtocol` in the AST for
a certain scope.
Checking that the constant is not written to after being initialized is
supposed to happen in the Mandatory SILOptimizer phase instead.

On loads, the Optimizer checks, that a variable is fully initialized,
but perviously did not validate what happens to it after it is loaded.
This allowed loading the memory object through an open_existential_addr
instruction and applying the result as an Operand to a mutating witness
method.