[MS] Zero out ECX in __cpuid in intrin.h

Summary:
Some CPUID leafs depend on the value of ECX as well as EAX, but we left
it uninitialized.

Originally reported as https://crbug.com/901547

Reviewers: craig.topper, hans

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D54171

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@346265 91177308-0d34-0410-b5e6-96231b3b80d8