Adding originatingElement to generated JavaBuild

[mtraynham]

MapStruct 1.4.0.Final was released recently, so I was hoping to take advantage of the full benefits of Kapt annotation incremental compiling for faster builds, but it didn't seem to be working and always seemed to do a full build.

The Builder annotation processing via mapstruct-kotlin had this spurious line in the logs during the build,

[WARN] Issue detected with com.github.pozo.BuilderProcessor. Expected 1 originating source file when generating /project/build/generated/source/kapt/main/com/project/ItemBuilder.java, but detected 0: [].

I tracked down the error message to Jetbrains kapt, https://github.com/JetBrains/kotlin/blob/master/plugins/kapt3/kapt3-base/src/org/jetbrains/kotlin/kapt3/base/incremental/incrementalProcessors.kt#L126-L129. Interestingly, if you hit this, it seems to set that isFullRebuild = true, which is probably not ideal.

Adding the originatingElement data class to the TypeSpec.classBuilder seems to correct the issue.

[mtraynham]

@Pozo would be great if this got merged, seems to reduce build times immensely!