[GlobalObject] Fix setSection("")

Summary:
In rL291613, the section name was interned in LLVMContext. However,
this broke the ability to remove the section from a GlobalObject,
because it tried to intern empty strings, which is not allowed.
Fix that and add an appropriate regression test.

Reviewed By: rnk
Differential Revision: https://reviews.llvm.org/D29795

llvm-svn: 295238