Revert 374967 "[Concepts] ConceptSpecializationExprs mangling"

This reverts commit 5e34ad10.

The mangling test fails on Windows:
http://lab.llvm.org:8011/builders/clang-x64-ninja-win7/builds/15944

It also fails on ppc64le:
http://lab.llvm.org:8011/builders/clang-ppc64le-linux-lnt/builds/21092

Also revert follow-up  374971 "Fix failing mangle-concept.cpp test."
(it did not help on Win/ppc64le).

llvm-svn: 374985