CodeGen: correct mangling for blocks

This addresses a regression introduced with SVN r219393.  A block may be
contained within another block.  In such a scenario, we would end up within a
BlockDecl, which is not a NamedDecl (as the names are synthesised).  The cast to
a NamedDecl of the DeclContext would then assert as the types are unrelated.

Restore the mangling behaviour to that prior to SVN r219393.  If the current
block is contained within a BlockDecl, walk up to the parent DeclContext,
recursively, until we have a non-BlockDecl.  This is expected to be a NamedDecl.
Add in a couple of asserts to ensure that the assumption that we only encounter
a block within a NamedDecl or a BlockDecl.

llvm-svn: 219696