[DebugInfo] Re-engineer a test to be stricter, add XFails

In the LLVM-IR for this test, the inlined argument "b" in the "a" function
is optimized out on certain architectures, not on others. This hasn't been
reported as a test failure since 93faeecd8fa and ff2073a51 because we would
create a variable that looks like this:

  DW_TAG_formal_parameter
    DW_AT_abstract_origin

With no further information (and no location). With D95617 however, we
stop emitting such variables.

Prior to landing D95617: make this test stricter by checking that the
variable mentioned above has a location. We have to accept that on certain
architectures this goes missing, so add those to the XFail list.

I've run a few experiments, and right now it looks likely only powerpc64
still drops the variable location.

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