JVM_IR: delay writes to the source map until generateMethod

`generateMethodNode` should not have any side effects for the output to
be stable under incremental compilation.

jps-plugin/testData/incremental/incrementalJvmCompilerOnly/inlineFunctionSmapStability/b.kt

@@ -0,0 +1,7 @@
+package inline1
+
+import inline2.*
+
+fun g() = root()
+
+inline fun h() = root()