Fix several bugs in array -> pointer decomposition.

First, we got several CVR propagation cases wrong, which Eli pointed
out in PR2039.

Second, we didn't propagate address space qualifiers correctly, leading
to incorrect lowering of code in CodeGen/address-space.c.

Third, we didn't uniformly propagate the specifier in the array to the
pointer ("int[restrict 4]" -> "int *restrict").

This adds an ASTContext::getArrayDecayedType member that handles the 
non-trivial logic for this seemingly simple operation.

llvm-svn: 49078