Split out incomplete arrays from VariableArrayType into

IncompleteArrayType.  This should make code dealing with both incomplete 
and variable length arrays much more readable, plus it allows properly 
making the distinction between isVariableArrayType() and 
isVariablyModifiedType().  The patch is a little big, but it's 
strightforward. so I don't think there should be any issues.

llvm-svn: 47165