Generalise support for non-inheritable attributes

Inheritable attributes on declarations may be inherited by any later
redeclaration at merge time.  By contrast, a non-inheritable attribute
will not be inherited by later redeclarations.  Non-inheritable
attributes may be semantically analysed early, allowing them to
influence the redeclaration/overloading process.

Before this change, the "overloadable" attribute received special
handling to be treated as non-inheritable, while all other attributes
were treated as inheritable.  This patch generalises the concept,
while removing a FIXME.  Some CUDA location attributes are also marked
as non-inheritable in order to support special overloading semantics
(to be introduced in a later patch).

The patch introduces a new Attr subclass, InheritableAttr, from
which all inheritable attributes derive.  Non-inheritable attributes
simply derive from Attr.

N.B. I did not review every attribute to determine whether it should
be marked non-inheritable.  This can be done later on an incremental
basis, as this change does not affect default functionality.

llvm-svn: 123959