[AST] Don't consider 'ExportDecl' when calculating DeclContext 'Encloses'

This mimics the style of 90010c2e (Don't consider 'LinkageSpec' when
calculating DeclContext 'Encloses'). Since ExportDecl and LinkageSpec
are transparent DeclContext, they share some similarity.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D116911