[llvm] [DebugInfo][DWARF] Set is_stmt on first non-line-0 instruction in BB (PR #105524)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

I added the test to the `Generic/` test dir since it doesn't test target-specific behaviour, but it does rely on the basic block layout which varies between targets; added commit 616f7d3d4f6d9bea6f776e357c938847e522a681 to move the test into `X86/`, which should fix the buildbot errors.

https://github.com/llvm/llvm-project/pull/105524