[CodeGen] fix const-ness of builtin equivalents of <math.h> and <complex.h> functions...

[CodeGen] fix const-ness of builtin equivalents of <math.h> and <complex.h> functions that might set errno

This just makes const-ness of the builtins match const-ness of their lib function siblings.
We're deferring fixing some of these that are obviously wrong to follow-up patches.
Hopefully, the bugs are visible in the new test file (added at rL317220).

As the description in Builtins.def says: "e = const, but only when -fmath-errno=0".

This is step 2 of N to fix builtins and math calls as discussed in D39204.

Differential Revision: https://reviews.llvm.org/D39481

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@317265 91177308-0d34-0410-b5e6-96231b3b80d8