Finally fix PR2189.  This makes a fairly invasive but important change to
move getAsArrayType into ASTContext instead of being a method on type.
This is required because getAsArrayType(const AT), where AT is a typedef
for "int[10]" needs to return ArrayType(const int, 10).

Fixing this greatly simplifies getArrayDecayedType, which is a good sign.

llvm-svn: 54317