[clang] Fix missing contract flag in sqrt intrinsic

The fp options specified through pragma are already encoded in Expr.

This patch takes the same approach used by clang codegen to emit
fastmath flags for fadd insts, basically use RAII to set the
current fastmath flags in IRBuilder, which is then used to emit
sqrt intrinsic.

Fixes: https://github.com/llvm/llvm-project/issues/64653