[Symbol] Give ClangASTContext a PersistentExpressionState instead of a ClangPersistentVariables

ClangASTContext doesn't use m_persistent_variables in a way specific to
ClangPersistentVariables. Therefore, it should hold a unique pointer to
PersistentExpressionState instead of a ClangPersistentVariablesUP.
This also prevents you from pulling in a plugin header when including
ClangASTContext.h

Doing this exposed an implicit dependency in ObjCLanguage that was
corrected by including ClangModulesDeclVendor.h

llvm-svn: 371470