Update `SourceManager::ContentCache::OrigEntry` to keep the original `FileEntryRef`, and use that to enable `ModuleMap::getModuleMapFile*` to return the original `FileEntryRef`. This change should be NFC for most users of `SourceManager::ContentCache`, but it could affect behaviour for users of `getNameAsRequested` such as in `compileModuleImpl`. I have not found a way to detect that difference without additional functional changes, so there is no test change here. Note: this should fix the Windows failure on https://reviews.llvm.org/D134923 which was caused by an incidental `getFile` call mutating `LastRef` on the `FileEntry`.

Update `SourceManager::ContentCache::OrigEntry` to keep the original `FileEntryRef`, and use that to enable `ModuleMap::getModuleMapFile*` to return the original `FileEntryRef`. This change should be NFC for most users of `SourceManager::ContentCache`, but it could affect behaviour for users of `getNameAsRequested` such as in `compileModuleImpl`. I have not found a way to detect that difference without additional functional changes, other than incidental cases like changes from / to \ on Windows so there is no new test. Note: this should fix the Windows failure on https://reviews.llvm.org/D134923 which was caused by an incidental `getFile` call mutating `LastRef` on the `FileEntry`.

Update `SourceManager::ContentCache::OrigEntry` to keep the original `FileEntryRef`, and use that to enable `ModuleMap::getModuleMapFile*` to return the original `FileEntryRef`. This change should be NFC for most users of `SourceManager::ContentCache`, but it could affect behaviour for users of `getNameAsRequested` such as in `compileModuleImpl`. I have not found a way to detect that difference without additional functional changes, other than incidental cases like changes from / to \ on Windows so there is no test change herenew test. Note: this should fix the Windows failure on https://reviews.llvm.org/D134923 which was caused by an incidental `getFile` call mutating `LastRef` on the `FileEntry`.