Driver: Set TOOL_NO_EXPORTS when building a "production" compiler, we don't

currently expect that to be useful for plugins, and this is important for
startup performance:
--
ddunbar@lordcrumb:tmp$ touch empty.c
ddunbar@lordcrumb:tmp$ runN 100 ~/llvm.obj.64/Release/bin/clang -c empty.c
name	  avg  	  min  	  med  	  max  	   SD  	 total 
user	 0.0054	 0.0052	 0.0054	 0.0055	 0.0000	 0.5350
 sys	 0.0084	 0.0090	 0.0078	 0.0087	 0.0008	 0.8390
wall	 0.0149	 0.0149	 0.0149	 0.0149	 0.0003	 1.4943
ddunbar@lordcrumb:tmp$ runN 100 ~/llvm.obj.64/Release/bin/clang -c empty.c
name	  avg  	  min  	  med  	  max  	   SD  	 total 
user	 0.0036	 0.0036	 0.0036	 0.0038	 0.0000	 0.3646
 sys	 0.0072	 0.0071	 0.0068	 0.0070	 0.0006	 0.7158
wall	 0.0123	 0.0123	 0.0122	 0.0136	 0.0003	 1.2262
--

llvm-svn: 113638