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
This is controlled by a new isWholeModule() attribute in SILModule.
It gives about 9% code size reduction on the benchmark executables.
For test-suite reasons it is currently not done for the stdlib.
Swift SVN r22491
