Make cppcoreguidelines-pro-type-member-init-use-assignment.cpp pass on...

Make cppcoreguidelines-pro-type-member-init-use-assignment.cpp pass on platforms where char is unsigned

The other options are to completely specify the triple (reduces test
coverage), or to specify a regex that allows either '0' or '0U' for char
initializers, however, that relaxes the test.

llvm-svn: 361629