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This *does not* contain the test files themselves. Those will be added in a
future commit.

This only contains the cmake files needed to build and the tests, the license
files that govern the tests themselves and changes to the existing
infrastructure. A README and other documentation has been provided.

Some highlights:

  - The tests have only been enabled on non-Windows, non-Apple, x86 and Aarch64
    platforms.

  - The gfortran tests are largely divided into "compile" tests that exercise
    specific behaviors of the compiler such as error messages and the ability
    to adhere to the standard, and "execute" tests that test the end-to-end
    behavior of the compiler and the correctness of the resulting binary.
    In this patch, only the "execute" tests are enabled.

  - A number of the tests have been disabled because they either trigger a
    "not yet implemented" assertion in flang, they cause flang to crash for
    a different reason (due to bugs/unimplemented features not guarded by an
    unimplemented assertion), they use non-standard extensions not (currently)
    supported by flang, or if they fail at runtime (likely because of bugs
    in the compiler or flang runtime, or because of unimplemented features in
    the runtime).

  - The test files themselves will be governed by GPL. The files committed in
    this patch will be governed by the LLVM license.

Differential Revision: https://reviews.llvm.org/D146485