I usually would fold this into the assert, but the comment there
suggests side effects. NFC.
ModuleMap.cpp:938:9: error: unused variable 'MainFile' [-Werror,-Wunused-variable]
auto *MainFile = SourceMgr.getFileEntryForID(SourceMgr.getMainFileID());
// Mark the main source file as being within the newly-created module so that
// declarations and macros are properly visibility-restricted to it.
auto *MainFile = SourceMgr.getFileEntryForID(SourceMgr.getMainFileID());
+ (void)MainFile;
assert(MainFile && "no input file for module implementation");
return Result;