Until now, `-x` wasn't really taken into account in Flang's compiler and
frontend drivers. `flang-new` and `flang-new -fc1` only recently gained
powers to consume inputs other than Fortran files and that's probably
why this hasn't been noticed yet.
This patch makes sure that `-x` is supported correctly and consistently
with Clang. To this end, verification is added when reading LLVM IR
files (i.e. IR modules are verified with `llvm::verifyModule`). This
way, LLVM IR parsing errors are correctly reported to Flang users. This
also aids testing.
With the new functionality, we can verify that `-x ir` breaks
compilation for e.g. Fortran files and vice-versa. Tests are updated
accordingly.
Differential Revision: https://reviews.llvm.org/D127207
