Change the JIT to compile eagerly by default as agreed in

Change the JIT to compile eagerly by default as agreed in
http://llvm.org/PR5184, and beef up the comments to describe what both options
do and the risks of lazy compilation in the presence of threads.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85295 91177308-0d34-0410-b5e6-96231b3b80d8