Introduce updateDiscriminator interface to DILocation to make it cleaner assigning discriminators.

Summary: This patch introduces updateDiscriminator to DILocation so that it can be directly called by AddDiscriminator. It also makes it easier to update the discriminator later.

Reviewers: dnovillo, dblaikie, aprantl, echristo

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D25959

llvm-svn: 285207