Remove assert that path exists on file system

This would trigger when path prefixes are remapped, as the buffer
identifier is the remapped path, not the path on the file system. There
is no easy way to recover the original path, so drop the assert.

lib/ClangImporter/ClangSourceBufferImporter.cpp

@@ -58,7 +58,6 @@ SourceLoc ClangSourceBufferImporter::resolveSourceLocation(
     // different contents in different modules, despite the same name.
     auto IDOpt = swiftSourceManager.getIDForBufferIdentifier(bufIdent);
     if (IDOpt.has_value() && clangSrcMgr.getFileEntryForID(clangFileID)) {
-      CONDITIONAL_ASSERT(llvm::sys::fs::exists(bufIdent));
       mirrorID = IDOpt.value();
     } else
       mirrorID = swiftSourceManager.addNewSourceBuffer(std::move(mirrorBuffer));