[LLVM-C] Add target triple normalization to the C API.

rL333307 was introduced to remove automatic target triple
normalization when calling sys::getDefaultTargetTriple(), arguing
that users of the latter already called Triple::normalize()
if necessary. However, users of the C API currently have no way of
doing target triple normalization.

This patch introduces an LLVMNormalizeTargetTriple function to
the C API which wraps Triple::normalize() and can be used on
the result of LLVMGetDefaultTargetTriple to achieve the same effect.

Differential Revision: https://reviews.llvm.org/D49414

Reviewed By: whitequark

llvm-svn: 337263