Nominate Erich Keane for OpenACC

Erich is the driving force behind the OpenACC implementation work that
has recently begun in Clang. Given his expertise on the topic and that
he's already aware of maintainer expectations (he maintains templates
and attributes currently), we should recognize that he's also the one
maintaining OpenACC.

Author: AaronBallman

clang/Maintainers.rst

@@ -244,6 +244,12 @@ OpenCL conformance
 | anastasia\@compiler-experts.com (email), Anastasia (Phabricator), AnastasiaStulova (GitHub)
 
 
+OpenACC
+~~~~~~~
+| Erich Keane
+| ekeane\@nvidia.com (email), ErichKeane (Phabricator), erichkeane (GitHub)
+
+
 SYCL conformance
 ~~~~~~~~~~~~~~~~
 | Alexey Bader