[Clang][ConstExpr] Allow __builtin_elementwise_sqrt + __builtin_sqrt/sqrtf/sqrtl to be used in constant expressions

There's quite a bit of work to do before we can handle this, including some of the libc support - its not as easy as hooking up to a APFloat::sqrt method.

This also might need splitting as we lower the elementwise to ` @llvm.sqrt.XXXX` but the scalar builtins typically to libcalls

No files or tests are named. Start by tracing Clang constant-expression handling for __builtin_elementwise_sqrt and the scalar sqrt builtins, then examine the APFloat::sqrt and libc support mentioned in the issue. Check how elementwise calls lower to @llvm.sqrt while scalar builtins lower to libcalls; done requires constant-expression support for the requested builtins.

Written by the indexing model from the issue text.