[llvm-profdata] Add support for weighted merge of profile data

This change adds support for an optional weight when merging profile data with the llvm-profdata tool.
Weights are specified by adding an option ':<weight>' suffix to the input file names.

Adding support for arbitrary weighting of input profile data allows for relative importance to be placed on the
input data from multiple training runs.

Both sampled and instrumented profiles are supported.

Reviewers: dnovillo, bogner, davidxl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14547

llvm-svn: 254669