Strips the first lines destination lines from the sourcemap, decrementing all remaining
destination line numbers accordingly. Used to re-anchor a sourcemap after removing a
prefix (e.g. a shebang line) from the generated code.
Like collapse_sourcemaps, but keeps only the sources and names that a traced token
references, as Rollup’s traceMappings() does. So a map with no mappings (e.g. a transform’s
{ mappings: '' }) cuts out the code before it. A chain of one map is traced too, but it
keeps its other fields, such as sourceRoot, as getCombinedSourcemap did before.