[build.py] Add -fms-compatibility-version=19 to build.py
clang-cl can't compile tests containing char16_t and char32_t types
without the MSVC compatibility option passed. This patch adds the option to the
clang-cl call in the build.py script by default.
Reviewers: zturner, labath, stella.stamenova, serge-sans-paille
Reviewed By: labath
Subscribers: lldb-commits, leonid.mashinskiy
Differential Revision: https://reviews.llvm.org/D57809