[AutoFDO] Make call targets order deterministic for sample profile

Summary:
StringMap is used for storing call target to frequency map for AutoFDO. However the iterating order of StringMap is non-deterministic, which leads to non-determinism in AutoFDO profile output. Now new API getSortedCallTargets and SortCallTargets are added for deterministic ordering and output.

Roundtrip test for text profile and binary profile is added.

Reviewers: wmi, davidxl, danielcdh

Subscribers: hiraditya, mgrang, llvm-commits, twoh

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66191

llvm-svn: 369440