[sanitizer] Intercept weak functions in dll_thunks.

In this diff, I update current implementation of the interception in dll_thunks
to consider the special case of weak functions.
First we check if the client has redefined the function in the main executable
(for example: __sanitizer_cov_trace_pc_guard). It we can't find it, then we look
for the default implementation (__sanitizer_cov_trace_pc_guard__dll). The
default implementation is always available because the static runtime is linked
to the main executable.

Differential Revision: https://reviews.llvm.org/D29155

llvm-svn: 293952