Add test coverage to printing of enums and fix display of unsigned values

TestCPP11EnumTypes.py should have covered all our bases when it comes
to typed enums, but it missed the regression introduced in r374066.
The reason it didn't catch it is somewhat funny: the test was copied
over from another test that recompiled a source file with a different
base type every time, but neither the test source nor the python code
was adapted for testing enums. As a result, this test was just running
8 times the exact same checks on the exact same binary.

This commit fixes the coverage and addresses the issue revealed by
the new tests.

llvm-svn: 374108