[ORC] Rename SymbolSource to MaterializationUnit, and make the materialization

operation all-or-nothing, rather than allowing materialization on a per-symbol
basis.

This addresses a shortcoming of per-symbol materialization: If a
MaterializationUnit (/SymbolSource) wants to materialize more symbols than
requested (which is likely: most materializers will want to materialize whole
modules) then it needs a way to notify the symbol table about the extra symbols
being materialized. This process (checking what has been requested against what
is being provided and notifying the symbol table about the difference) has to
be repeated at every level of the JIT stack. Making materialization
all-or-nothing eliminates this issue, simplifying both materializer
implementations and the symbol table (VSO class) API. The cost is that
per-symbol materialization (e.g. for individual symbols in a module) now
requires multiple MaterializationUnits.

llvm-svn: 327946