[flang][driver] Add missing parentheses in an assert

[awarzynski]

Summary

The assert in https://reviews.llvm.org/D124665 was missing parentheses,
which triggered a warning in GCC (verified with GCC 11). As -Werror is
on by default in FLang, that triggered build errors, see e.g. [1].

The fix is rather straightforward, so I am sending this without a
review.

[1] https://lab.llvm.org/buildbot/#/builders/160/builds/7016

flang/lib/Frontend/FrontendActions.cpp

 /// \param [in] act Backend act to run (assembly vs machine-code generation)
 /// \param [in] llvmModule LLVM module to lower to assembly/machine-code
 /// \param [out] os Output stream to emit the generated code to
 static void GenerateMachineCodeOrAssemblyImpl(clang::DiagnosticsEngine &diags,
                                               llvm::TargetMachine &TM,
                                               BackendActionTy act,
                                               llvm::Module &llvmModule,
                                               llvm::raw_pwrite_stream &os) {
-  assert((act == BackendActionTy::Backend_EmitObj) ||
-         (act == BackendActionTy::Backend_EmitAssembly) &&
-             "Unsupported action");
+  assert(((act == BackendActionTy::Backend_EmitObj) ||
+          (act == BackendActionTy::Backend_EmitAssembly)) &&
 
   // Set-up the pass manager, i.e create an LLVM code-gen pass pipeline.
   // Currently only the legacy pass manager is supported.
   // TODO: Switch to the new PM once it's available in the backend.
   llvm::legacy::PassManager CodeGenPasses;
   CodeGenPasses.add(
       createTargetTransformInfoWrapperPass(TM.getTargetIRAnalysis()));