Rename the new `_GUID` clang module to `_GUIDDef`. (#84466)
This PR changes the name of the clang module introduced in #83914 from
`_GUID` to `_GUIDDef`. This change is necessary because the Windows SDK
declares a type named `_GUID` and Swift gets confused when a module and
a type have the same name.
This confusion is the motivation for
[SE-0491](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0491-module-selectors.md),
but we can't just wait for a fix because the new module is a
source-breaking change and clients won't be able to adopt the proposed
syntax for some time.
