Go binding: Add GetCurrentDebugLocation to obtain debug location from builder

Summary:

Currently Go binding only has SetCurrentDebugLocation method.
I added GetCurrentDebugLocation method to IRBuilder instance.

I added this because I want to save current debug location, change debug location temporary and restore the saved one finally.
This is useful when source location jumps and goes back after while LLVM IR generation.

I also added tests for this to ir_test.go.
I confirmed that all test passed with this patch based on r298890

Patch by Ryuichi Hayashida!

Differential Revision: https://reviews.llvm.org/D31415

llvm-svn: 299185