Create msbuild only when using MSVC

Summary:
I could be wrong but it seems to have no use for MinGW.

Related diff: https://reviews.llvm.org/D29772

Reviewers: chandlerc, rnk

Reviewed By: rnk

Patch by: Mateusz Mikuła

Subscribers: rnk, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D29952

llvm-svn: 297985