[clang][NFC] Remove IgnoreLinkageSpecDecls

The Itanium mangler uses IgnoreLinkageSpecDecls to strip linkage spec
contexts.  It doesn't do this consistently, but there is no need for
it to do it at all.  getEffectiveDeclContext never returns a linkage
spec, as it either recurses, uses getRedeclContext (which itself
removes the specs), or gets the decl context of non-namespace entities.

This patch removes the function and all calls to it.  For safety I add
a couple of asserts to make sure we never get them.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D119748