COFF: Make symbols satisfy weak ordering.

Previously, SymbolBody::compare(A, B) didn't satisfy weak ordering.
There was a case that A < B and B < A could have been true.
This is because we just pick LHS if A and B are consisdered equivalent.

This patch is to make symbols being weakly ordered. If A and B are
not tie, one of A < B && B > A or A > B && B < A is true.
This is not an improtant property for a single-threaded environment
because everything is deterministic anyways. However, in a multi-
threaded environment, this property becomes important.

If a symbol is defined or lazy, ties are resolved by its file index.
For simple types that we don't really care about their identities,
symbols are compared by their addresses.

llvm-svn: 241294