When importing a C++ struct that contains two methods that only differ in const-ness, we append `Mutating` to the name of the non-const method to make it possible to call from Swift unambiguously.
Unfortunately that logic was dependent on the order in which we import methods of a class: the `Mutating` suffix was added when another method with the same name was already imported.
This caused lookup failures, and the behavior was incorrect when the pair of methods return instances of an unsafe type: the const overload was renamed as `Unsafe` properly, but the non-const overload was not renamed.
C++ standard library module is called `std`. To make it more clear to a Swift developer that this module is a C++ stdlib, and not a Swift stdlib, let's rename it to `CxxStdlib`.
This is the first step in the module rename. We don't ban `std` in this patch to be able to build SwiftCompilerSources with hosttools until a new Swift compiler is shipped.
