JVM: separate the two kinds of source mappers

* a writing source mapper has `mapLineNumber(line, file, class)` that
   inserts a new SMAP entry and returns a fake line number from it;
 * a copying source mapper has `mapLineNumber(line)` that uses an
   existing SMAP to resolve the line number and call the former method
   on a different source mapper;
 * those two types are disjoint.

compiler/testData/codegen/boxInline/smap/anonymous/severalMappingsForDefaultFile.kt

@@ -74,7 +74,8 @@ test/_1Kt
 _2Kt
 *L
 1#1,18:1
-14#2,2:19
-10#2:21
-7#3:22
+14#2:19
+10#2:20
+15#2:22
+7#3:21
 *E

compiler/testData/codegen/boxInline/smap/newsmap/mappingInInlineFunLambda.kt

@@ -90,6 +90,7 @@ test/_1Kt
 _2Kt
 *L
 1#1,19:1
-7#2,3:20
-9#3,2:23
+7#2,2:20
+9#2:24
+9#3,2:22
 *E