swift234

Reapply "stdlib: libm's sqrt and llvm.sqrt are not semantically equivalent""

This reapplies commit r22864 - it is not changing the public api as we initially
thought. sqrt() was never available without importing Darwin.

This change only changes where sqrt() gets "forwarded" to. Before 'sqrt' called
the builtin '_sqrt' defined in BuiltinMath now it just calls the math library's
'sqrt' function.

I also added a stdlib test.

rdar://18371371

Swift SVN r22870