[MIRTesting] Abort when failing to parse a function.

[MIRTesting] Abort when failing to parse a function.

When we failed to parse a function in the mir parser, we should abort
the whole compilation instead of continuing in a weird state. Indeed,
this was creating strange machine function passes failures that were
hard to understand, until we notice that the function actually did not
get parsed correctly!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@276348 91177308-0d34-0410-b5e6-96231b3b80d8