[clang] [NFC][clang][Driver] Add tests for --driver-mode=flang (PR #207658)

[Yusuke MINATO via cfe-commits]

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@@ -5,3 +5,10 @@
 ! CHECK-SYNTAX-ONLY: "{{[^"]*}}/Inputs/one.f90"
 ! CHECK-SYNTAX-ONLY-LABEL: "{{[^"]*}}clang{{[^"/]*}}" "-cc1"
 ! CHECK-SYNTAX-ONLY: "{{[^"]*}}/Inputs/other.c"
+
+! RUN: not %clang --driver-mode=flang -### -Xflang -std=f2018 %S/Inputs/one.f90 -Xclang -std=c17 %S/Inputs/other.c 2>&1 | FileCheck --check-prefixes=MIXED-OPT %s
+! MIXED-OPT: clang: error: unknown argument '-Xclang'
+! MIXED-OPT-NOT: "{{[^"]*}}flang{{[^"/]*}}" "-fc1"
+! MIXED-OPT-NOT: "-std=f2018"
+! MIXED-OPT-NOT: "{{[^"]*}}clang{{[^"/]*}}" "-cc1"
+! MIXED-OPT-NOT: "-std=c17"
----------------
yus3710-fj wrote:

I am considering verifying that the driver invokes the appropriate frontends for each file and does not distribute flags intended for them.
As you mentioned, checking both the error message and the absence of the invocations is redundant. However, I would prefer to remove the check for the error message. What are your thoughts on this?

https://github.com/llvm/llvm-project/pull/207658