[clang] Add tests for CWG issues 977, 1482, 2516

CWG977 focus on point of /completeness/ of enums. Wording provided in CWG1482.
CWG1482 and CWG2516 focus on locus (point) of /declaration/. Wording provided in CWG2516.

Reviewed By: #clang-language-wg, shafik

Differential Revision: https://reviews.llvm.org/D151042