IRBuilder: Remove redundant check around SetInstDebugLocation call.
PR14566: Debug Info: Removing top level lexical blocks

This adds support for LLVM to accept metadata that doesn't include a top level
lexical block in a function. Specifically LLVM couldn't handle this when there
were file changes relating to these blocks. I've updated a few test cases to
ensure other functionality (such as inlining) isn't affected by this change, but
haven't pervasively updated all the test cases.