[support] remove tautological comparison in Support/Windows/Path.inc

Summary:
The removed code checks that we are able to handle a 64-bit number, but
the code we're calling takes two dwords (for a total of 64 bits), so this
is always true.

Reviewers: zturner, rnk, majnemer, compnerd

Reviewed By: zturner

Subscribers: amccarth, hiraditya, lebedev.ri, llvm-commits

Differential Revision: https://reviews.llvm.org/D39263

llvm-svn: 316814