[flang] [clang] [flang][Driver] Let the linker fail on multiple definitions of main() (PR #73124)

[Michael Klemm via cfe-commits]

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+! RUN: %clang -x c -o %t.c-part -c %s.c-part
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mjklemm wrote:

That does not seem to work and gives me this error when running the test.

`error: unknown integrated tool '-cc1'. Valid tools include '-fc1'.`

I do not necessarily need `clang` as the C compiler for the test.  GCC (or whatever was used to build that instance of LLVM) would also be sufficient.

I guess that problem will come back when testing `BIND(C)`.  So, maybe a more generic and better solution might be needed.  But I guess that may be beyond this PR.

https://github.com/llvm/llvm-project/pull/73124