[HotColdSplit] With PGO add profile entry metadata to split cold function

Summary:
When compiling with profile data, ensure the split cold function gets
cold function_entry_count metadata (just use 0 since it should be cold).
Otherwise with function sections it will not be placed in the unlikely
text section with other cold code.

Reviewers: vsk

Subscribers: sebpop, hiraditya, davidxl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57900

llvm-svn: 353434