Add TypedViewRegion::isBoundable() to indicate whether or not the

TypedViewRegion has a valid rvalue type. Also renamed instance variable 'T' to
'LvalueType' to make it unambiguous of its purpose.

This fixes some crashes I was seeing after:

http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20090309/013771.html

This is because 'isBoundable()' is defined in TypedRegion (the parent class) in
terms of the rvalue type (which could be null), while for TypedViewRegion it
should be defined in terms of the lvalue type.

llvm-svn: 66712