This change introduces three new issue templates for DXC. The three
issue templates are:
* feature_request - Issues filed with this template will get
automatically tagged with the `enhancement` label
* bug_report - Issues filed with this template will get automatically
get tagged with the `bug` label
* bug_report_spirv - Issues filed with this template will get
automatically tagged with the `bug` and `spirv` labels
The bug_report and bug_report_spirv templates are otherwise identical,
but could diverge to better meet the needs of the supporting teams.
Fixes#4975.