Fix msan issue that caused revert in
https://github.com/llvm/llvm-project/pull/135695
### Original message
Now that LLVM dialect has `blockaddress` support, introduce
import/translation for `indirectbr` instruction.
Add initial support for DIExpression in LLVM dialect.
Similar to LLVM IR, DI Expression is encoded as a list of uint64. The
difference is that LLVM IR has helpers for understanding the expression
(e.g. for verification and pretty printing), whereas the current support
added by this PR treats the expression elements as opaque.
The revision adds a flag to the LLVM IR import
that avoids emitting expensive warnings about
unsupported debug intrinsics and unhandled
metadata.
Reviewed By: Dinistro
Differential Revision: https://reviews.llvm.org/D153625
