[clang] [flang] [Flang][Driver] Add -print-resource-dir command line flag to emit Flang's resource directory (PR #90886)

[Michael Klemm via cfe-commits]

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+! RUN: %flang -print-resource-dir -resource-dir=%S/Inputs/resource_dir \
+! RUN:  | FileCheck -check-prefix=PRINT-RESOURCE-DIR -DFILE=%S/Inputs/resource_dir %s
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mjklemm wrote:

I'll make the variable a bit clearer in the next update.  That's a good suggestion, thanks!

As for `ClangExecutable`, I haven't looked at where this is being defined.  It's indeed the driver name and not the name of the Clang driver.  So, for Flang is actually contains the full path to `flang-new`.

I'm not opposed to renaming it as part of this PR, but it may widen the scope and make approval a bit more complex.  So, my feeling is that this refactoring should take place in a different PR.  But I can be convinced otherwise.

https://github.com/llvm/llvm-project/pull/90886