[llvm-profdata] Add support for weighted merge of profile data (2nd try)

Summary:
This change adds support for specifying a weight when merging profile data with the llvm-profdata tool.
Weights are specified by using the --weighted-input=<weight>,<filename> option. Input files not specified
with this option (normal positional list after options) are given a default weight of 1.

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: davidxl, dnovillo, bogner, silvas

Subscribers: silvas, davidxl, llvm-commits

Differential Revision: http://reviews.llvm.org/D15306

llvm-svn: 255659