Introduce support for "transparent" DeclContexts, which are

DeclContexts whose members are visible from enclosing DeclContexts up
to (and including) the innermost enclosing non-transparent
DeclContexts. Transparent DeclContexts unify the mechanism to be used
for various language features, including C enumerations, anonymous
unions, C++0x inline namespaces, and C++ linkage
specifications. Please refer to the documentation in the Clang
internals manual for more information.

Only enumerations and linkage specifications currently use transparent
DeclContexts.

Still to do: use transparent DeclContexts to implement anonymous
unions and GCC's anonymous structs extension, and, later, the C++0x
features. We also need to tighten up the DeclContext/ScopedDecl link
to ensure that every ScopedDecl is in a single DeclContext, which
will ensure that we can then enforce ownership and reduce the memory
footprint of DeclContext.

llvm-svn: 61735