Tolerate incorrect return type for 'isinf' and 'isnan' in tests.

Summary:
GLIBC recently removed the incorrect `int isinf(double)` and `int isnan(double)` overloads in C++11 and greater. This causes previously `XFAIL: linux`  tests to start passing.

Since there is no longer a way to 'XFAIL' the tests I choose to simply tolerate this bug.

See https://sourceware.org/bugzilla/show_bug.cgi?id=19439


Reviewers: rsmith, mclow.lists, EricWF

Subscribers: jroelofs, cfe-commits

Differential Revision: http://reviews.llvm.org/D19835

llvm-svn: 271060