Add a new way to bind a format to a type: by enum type

The "type format add" command gets a new flag --type (-t). If you pass -t <sometype>, upon fetching the value for an object of your type,
LLDB will display it as-if it was of enumeration type <sometype>
This is useful in cases of non-contiguous enums where there are empty gaps of unspecified values, and as such one cannot type their variables as the enum type,
but users would still like to see them as-if they were of the enum type (e.g. DWARF field types with their user-reserved ranges)

The SB API has also been improved to handle both types of formats, and a test case is added

llvm-svn: 198105