[flang][driver] Add support for `-emit-mlir`

This patch adds support for generating MLIR files in Flang's frontend
driver (i.e. `flang-new -fc1`). `-emit-fir` is added as an alias for
`-emit-mlir`. We may want to decide to split the two in the future.

A new parent class for code-gen frontend actions is introduced:
`CodeGenAction`. We will be using this class to encapsulate logic shared
between all code-generation actions, but not required otherwise. For
now, it will:
 * run prescanning, parsing and semantic checks,
 * lower the input to MLIR.
`EmitObjAction` is updated to inherit from this class. This means that
the behaviour of `flang-new -fc1 -emit-obj` is also updated (previously,
it would just exit immediately). This change required
`flang/test/Driver/syntax-only.f90` to be updated.

For `-emit-fir`, a specialisation of `CodeGenAction` is introduced:
`EmitMLIRAction`. The key logic for this class is implemented in
`EmitMLIRAction::ExecuteAction`.

Differential Revision: https://reviews.llvm.org/D118985

1 files changed, 3 insertions, 0 deletions

diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index dcfa741..af59cb6 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -134,6 +134,9 @@ static bool ParseFrontendArgs(FrontendOptions &opts, llvm::opt::ArgList &args,
     case clang::driver::options::OPT_fsyntax_only:
       opts.programAction = ParseSyntaxOnly;
       break;
+    case clang::driver::options::OPT_emit_mlir:
+      opts.programAction = EmitMLIR;
+      break;
     case clang::driver::options::OPT_emit_obj:
       opts.programAction = EmitObj;
       break;