[sanitizer] if WINAPI is already defined, do not redefine it

Summary: lib/sanitizer_common/sanitizer_win_defs.h defineds WINAPI, which is also defined by standard Windows headers. Redefining it causes warnings during compilation. This change causes us to only define WINAPI if it is not already defined, which avoids the warnings.

Reviewers: rnk, zturner, mpividori

Reviewed By: rnk, mpividori

Subscribers: kubamracek

Differential Revision: https://reviews.llvm.org/D29683

llvm-svn: 294497