[RuntimeDyld][Orc][MCJIT] Add partial weak-symbol support to RuntimeDyld.

[RuntimeDyld][Orc][MCJIT] Add partial weak-symbol support to RuntimeDyld.

This patch causes RuntimeDyld to check for existing definitions when it
encounters weak symbols. If a definition already exists then the new weak
definition is discarded. All symbol lookups within a "logical dylib" should now
agree on the address of any given weak symbol. This allows the JIT to better
match the behavior of the static linker for C++ code.

This support is only partial, as it does not allow strong definitions that
occur after the first weak definition (in JIT symbol lookup order) to override
the previous weak definitions. Support for this will be added in a future
patch.

llvm-svn: 278065