[build.py] Add `-fms-compatibility-version=19` to `build.py`

Summary:
`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

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D57809

llvm-svn: 353709