[sanitizer] Make function declarations C-compatible

The public sanitizer headers are intended to be usable from either
C++ or C, but they declare no-argument functions with the syntax that
is not a proper prototype declaration in C. This goes unnoticed until
someone uses -Wsystem-headers.

Patch By: mcgrathr

Reviewers: phosek, vitalybuka

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D41475

llvm-svn: 321305