Use a DenseMap for looking up functions by UID in CompileUnit::FindFunctionByUID

Summary:
Instead of iterating over our vector of functions, we might as well use a map here to
directly get the function we need.

Thanks to Vedant for pointing this out.

Reviewers: vsk

Reviewed By: vsk

Subscribers: mgrang, lldb-commits

Differential Revision: https://reviews.llvm.org/D50225

llvm-svn: 339504