Add methods for LLVMTargetDataRef

[yowl]

This adds methods to LLVMTargetRef for some of the underlying LLVM api calls, covers the ones I want, struct element offsets, and others that I saw that took a LLVMTargetRef as the first parameter. I also added a class that wraps the LLVMTargetRef struct, LLVMTargetData thinking of #140 but not sure I've understood that right as doesn't seem to add much so let me know if that should come out. Added some simple tests for the new methods in LLVMTargetRef to at least make sure they return something sensible.

[yowl]

I've made some changes in line with the feedback, hopefully I'm getting closer. I went for option 2 and created a StructLayout class. It takes both a DataLayout and the StructType as it needs access to the LLVMTargetDataRef to call down. I dropped one of the methods CallFrameAlignmentOfType, as it ends up calling getABITypeAlignment as does ABIAlignmentOfType

 unsigned LLVMABIAlignmentOfType(LLVMTargetDataRef TD, LLVMTypeRef Ty) {
   return unwrap(TD)->getABITypeAlignment(unwrap(Ty));
 }
 
 unsigned LLVMCallFrameAlignmentOfType(LLVMTargetDataRef TD, LLVMTypeRef Ty) {
   return unwrap(TD)->getABITypeAlignment(unwrap(Ty));
 }

So in order to keep the name in line with the c name, I just dropped the duplicate.

[tannergooding]

Overall this LGTM now, minus the naming nits on methods.

[yowl]

Thanks!

[tannergooding]

Just wanted to make sure you saw (#141 (comment)):

Likewise, prefix with Get and similar for others below where it is a non-property and the C++ name is get*

I didn't explicitly provide a suggestion for all the names that should be updated and it looks like a couple still remain. https://llvm.org/doxygen/classllvm_1_1DataLayout.html has the C++ names if you need a reference.

[yowl]

Thanks, I'll look at the others.

[tannergooding]

Thanks!