[libcxx] Add support for LLVM_USE_SANITIZER=Undefined

LLVM_USE_SANITIZER=Undefined support was added to the LLVM CMake configuration.
Update libc++'s handling of LLVM_USE_SANITIZER to support this as well.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@219987 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' of http://llvm.org/git/libcxx
[libcxx] Fix SFINAE in <cmath>. Patch from K-Ballo.

Delay instantiation of `__numeric_type` within <cmath>, 
don't instantiate it when the `is_arithmetic` conditions do not hold as it causes
errors with user-defined types with ambiguous conversions. Fixes PR21083.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@219998 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' of http://llvm.org/git/libcxx