Add template type diffing to Clang. This feature will provide a better

comparison between two templated types when they both appear in a diagnostic.
Type elision will remove indentical template arguments, which can be disabled
with -fno-elide-type.  Cyan highlighting is applied to the differing types.

For more formatting, -fdiagnostic-show-template-tree will output the template
type as an indented text tree, with differences appearing inline. Template
tree works with or without type elision.

llvm-svn: 159216