Add support for DW_LNCT_LLVM_source #486

Job	Run time
Build fuzz targets	1m 20s
coverage	1m 6s
bench	1m 13s
build (ubuntu-latest, stable)	50s
build (ubuntu-latest, beta)	49s
cross (i686-unknown-linux-gnu)	48s
doc	38s
build (ubuntu-latest, nightly)	46s
features	2m 54s
cross (powerpc64-unknown-linux-gnu)	49s
build (stable, macOS-latest)	44s
msrv	1m 49s
msrv-read	1m 42s
rustfmt	7s
Run `debug_abbrev` fuzz target	5m 6s
Run `debug_aranges` fuzz target	5m 11s
Run `debug_info` fuzz target	5m 10s
Run `debug_line` fuzz target	5m 6s
Run `eh_frame` fuzz target	5m 10s
Run `eh_frame_hdr` fuzz target	5m 5s
	46m 23s

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