The implementation and intent behind freeing the triple string here is the same
as LLVMGetDefaultTargetTriple (and any other owned c string returned from the C
API), so we should use LLVMDisposeMessage for to free the string for
consistency.
Patch by Mats Larsen -- thanks Mats!
Reviewed By: lhames
Differential Revision: https://reviews.llvm.org/D102957