Cleanup some -Wundef warnings in include/llvm/Support/MathExtras.h
Cleanup some -Wundef warnings in include/llvm/Support/MathExtras.h

Fix avoids gratuitous warnings from gcc for "_MSC_VER" not being defined.

Differential Revision: http://reviews.llvm.org/D14598

Patch by Tony Kelman <tony@kelman.net>


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@253614 91177308-0d34-0410-b5e6-96231b3b80d8

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