[flang][cuda] Define _CUDA only when preprocessor is enabled (#155913)

From the CUDA Fortran programming guide: 
> If CUDA Fortran is enabled in compilation, either by specifying -⁠cuda
on the command line, and pre-processing is enabled by either the
-⁠Mpreprocess compiler option or by using capital letters in the
filename extension (.CUF, .F90, etc.) then the _CUDA macro is defined.

Move the definition of `_CUDA` to the compiler invocation.

1 files changed, 5 insertions, 0 deletions

diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index 4719a24..fb3a132 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -1754,6 +1754,11 @@ void CompilerInvocation::setDefaultPredefinitions() {
                                     fortranOptions.predefinitions);
   }
 
+  if (frontendOptions.features.IsEnabled(
+          Fortran::common::LanguageFeature::CUDA)) {
+    fortranOptions.predefinitions.emplace_back("_CUDA", "1");
+  }
+
   llvm::Triple targetTriple{llvm::Triple(this->targetOpts.triple)};
   if (targetTriple.isOSLinux()) {
     fortranOptions.predefinitions.emplace_back("__linux__", "1");