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

llvm/tools/msbuild/CMakeLists.txt

@@ -1,4 +1,4 @@
-if (WIN32)
+if (MSVC)
   # CPack will install a registry key in this format that we wish to reference.
   set(REG_KEY "${CPACK_PACKAGE_INSTALL_REGISTRY_KEY}")
   set(LIB_PATH_VERSION "${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}")