[msan] Intercept dladdr1, and refactor dladdr

This patch adds an msan interceptor for dladdr1 (with support
for RTLD_DL_LINKMAP and RTLD_DL_SYMENT) and an accompanying
test. It also adds a helper file, msan_dl.cpp, that contains
UnpoisonDllAddrInfo (refactored out of the dladdr interceptor)
and UnpoisonDllAddr1ExtraInfo.

Differential Revision: https://reviews.llvm.org/D154272