[InstrProf] Encapsulates access to AddrToMD5Map.

Summary:
This fixes a unittest failure introduced by D44717

D44717 introduced lazy sorting of the internal data structures of the
symbol table. The AddrToMD5Map getter was potentially exposing
inconsistent (unsorted) state. We could sort in the accessor, however,
a client may store the pointer and thus bypass the internal state
management of the symbol table. The alternative in this CL blocks
direct access to the state, thus ensuring consistent
externally-observable state.

Reviewers: davidxl, xur, eraman

Reviewed By: xur

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44757

llvm-svn: 328163