cache sourcemap lookups
cache sourcemap lookups

When resolving source map references, we must repeatedly
compute a relative path and then look up the matching SourceFile
object.  Using a single-entry cache helps a lot: in the example
build I was looking at it, it reduced this function from 10s to
2s.

PiperOrigin-RevId: 315566056

nawawi/closure-compiler,Yannic/closure-compiler,Yannic/closure-compiler,ChadKillingsworth/closure-compiler,vobruba-martin/closure-compiler,monetate/closure-compiler,monetate/closure-compiler,google/closure-compiler,nawawi/closure-compiler,vobruba-martin/closure-compiler,monetate/closure-compiler,google/closure-compiler,Yannic/closure-compiler,vobruba-martin/closure-compiler,ChadKillingsworth/closure-compiler,ChadKillingsworth/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,Yannic/closure-compiler,vobruba-martin/closure-compiler,nawawi/closure-compiler,google/closure-compiler,monetate/closure-compiler,google/closure-compiler