Revert r343589 "[clangd] Dex: add Corpus factory for iterators, rename, fold constant. NFC"

Declaring a field with the same name as a type causes GCC to error out:

Dex.h:104:10: error: declaration of 'clang::clangd::dex::Corpus clang::clangd::dex::Dex::Corpus' [-fpermissive]
   Corpus Corpus;
          ^
Iterator.h:127:7: error: changes meaning of 'Corpus' from 'class clang::clangd::dex::Corpus' [-fpermissive]
 class Corpus {

llvm-svn: 343610