rustc_trans: don't lose the cross-crate DefId, MIR trans needs it.

[eddyb]

We might have been missing out on some issues because MIR trans was never being used cross-crate.

cc @rust-lang/compiler

[rust-highfive]

r? @nrc

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[nikomatsakis]

r=me -- for the record, the idea is that if we use the inlined instance, we think the function is local, and then we don't find an entry in the mir-map, so using the original def-id solves that

[eddyb]

@bors r=nikomatsakis p=1 (so we don't have a nightly that's not always using MIR trans)

[bors]

📌 Commit ee977e7 has been approved by nikomatsakis

[bors]

⌛ Testing commit ee977e7 with merge 379bfd0...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-no-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-no-mir, auto-win-msvc-64-opt-rustbuild
Approved by: nikomatsakis
Pushing 379bfd0 to master...