Disable wrapping llvm-xray YAML output

Summary:
The YAML output produced by llvm-xray is supposed to be wrapped at the
arbitrary default of 70 columns set by `yaml:Output`.  Unfortunately,
the wrapping is rather unpredictable, and can easily go past the set
number of columns, depending on the execution environment.

To make the YAML output environment-independent, disable wrapping
instead.

Reviewers: dberris

Reviewed By: dberris

Subscribers: fhahn, llvm-commits

Differential Revision: https://reviews.llvm.org/D29962

llvm-svn: 295116