Fix Weak External symbol handling.

The fallback atom was used only when it's searching for a symbol in a library;
if an undefined symbol was not found in a library, the LLD looked for its
fallback symbol in the library.

Although it worked in most cases, because symbols with fallbacks usually occur
only in OLDNAMES.LIB (a standard library), that behavior was incompatible with
link.exe. This patch fixes the issue so that the semantics is the same as
MSVC's link.exe

The new (and correct, I believe) behavior is this:

 - If there's no definition for an undefined atom, replace the undefined atom
   with its fallback and then proceed (e.g. look in the next file or stop
   linking as usual.)

Weak External symbols are underspecified in the Microsoft PE/COFF spec. However,
as long as I observed the behavior of link.exe, this seems to be what we want
for compatibility.

Differential Revision: http://llvm-reviews.chandlerc.com/D2162

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195269 91177308-0d34-0410-b5e6-96231b3b80d8
Use NativeReferenceIvarsV2 if necessary.

NativeReferenceIvarsV1 cannot handle more than 65535 relocation targets
because its field to point to the target table is of type uint16_t. Because
of that limitation, the LLD couldn't link a file containing more than 65535
relocations. 65535 is not a big number - the LLD couldn't even link itself
with V1.

This patch solves the issue by adding NativeReferenceIvarsV2 support. The
new structure has more bits for the target table, so it can handle a large
number of relocatinos.

V2 structure is larger than V1. In order to prevent file bloating, V2 format
is used only when the resulting file cannot be represented in V1 format. The
writer and the reader support both V1 and V2 formats.

Differential Revision: http://llvm-reviews.chandlerc.com/D2217

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195270 91177308-0d34-0410-b5e6-96231b3b80d8