Add auto-exporting of symbols from tools so that plugins work on Windows

Description

Add auto-exporting of symbols from tools so that plugins work on Windows

The problem with plugins on Windows is that when building a plugin DLL it needs
to explicitly link against something (an exe or DLL) if it uses symbols from
that thing, and that thing must explicitly export those symbols. Also there's a
limit of 65535 symbols that can be exported. This means that currently plugins
only work on Windows when using BUILD_SHARED_LIBS, and that doesn't work with
MSVC.

This patch adds an LLVM_EXPORT_SYMBOLS_FOR_PLUGINS option, which when enabled
automatically exports from all LLVM tools the symbols that a plugin could want
to use so that a plugin can link against a tool directly. Plugins can specify
what tool they link against by using PLUGIN_TOOL argument to llvm_add_library.
The option can also be enabled on Linux, though there all it should do is
restrict the set of symbols that are exported as by default all symbols are
exported.

This option is currently OFF by default, as while I've verified that it works
with MSVC, linux gcc, and cygwin gcc, I haven't tried mingw gcc and I have no
idea what will happen on OSX. Also unfortunately we can't turn on
LLVM_ENABLE_PLUGINS when the option is ON as bugpoint-passes needs to be
loaded by both bugpoint.exe and opt.exe which is incompatible with this
approach. Also currently clang plugins don't work with this approach, which
will be fixed in future patches.

Differential Revision: http://reviews.llvm.org/D18826