Patch allows to pass a symbols file to linker.
LLD will map symbols to sections and sort sections
in output according to symbol ordering file.

That can help to reduce the startup time and/or
amount of pagefaults during startup.

Also, interesting benchmark result was produced by Rafael Espíndola. 
After applying the symbols file for clang he timed compiling 
X86MCTargetDesc.ii to an object file.  

The page faults went from just
56,988 to 56,946 since most faults are not in the binary.
Running time went from 4.403053515 to 4.178112244. 
The speedup seems to be because of better cache
locality.

Differential revision: https://reviews.llvm.org/D26130

llvm-svn: 286440