[DebugInfo][InstrRef] Handle llvm.frameaddress intrinsics gracefully

When working out which instruction defines a value, the
instruction-referencing variable location code has a few special cases for
physical registers:
 * Arguments are never defined by instructions,
 * Constant physical registers always read the same value, are never def'd

This patch adds a third case for the llvm.frameaddress intrinsics: you can
read the framepointer in any block if you so choose, and use it as a
variable location, as shown in the added test.

This rather violates one of the assumptions behind instruction referencing,
that LLVM-ir shouldn't be able to read from an arbitrary register at some
arbitrary point in the program. The solution for now is to just emit a
DBG_PHI that reads the register value: this works, but if we wanted to do
something clever with DBG_PHIs in the future then this would probably get
in the way. As it stands, this patch avoids a crash.

Differential Revision: https://reviews.llvm.org/D106659

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