I was trying to add .cfi_ annotations to assembly code in the FreeBSD
kernel and changed a macro that then resulted in incorrectly nested
directives. However, clang's diagnostics said the error was happening at
<unknown>:0, so this patch requires a SMLoc to be passed along to all
callers of MCStreamer::getCurrentDwarfFrameInfo(). This addresses one of
the TODOs added in D51695.