Revert "[clang] Instantiate concepts with sugared template arguments"

Revert "[clang] Instantiate concepts with sugared template arguments"

This reverts commit b8064374b217db061213c561ec8f3376681ff9c8.

Based on the report here:
https://github.com/llvm/llvm-project/issues/59271

this produces a significant increase in memory use of the compiler and a
large compile-time regression.  This patch reverts this so that we don't
branch for release with that issue.