[rebase] include llvm FileSystem.h header for getMainExecutable in bbc

Author: jeanPerier

flang/tools/bbc/bbc.cpp

@@ -49,6 +49,7 @@
 #include "mlir/Transforms/Passes.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ErrorOr.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"