Do not generate LLVM IR for available_externally function bodies at

Do not generate LLVM IR for available_externally function bodies at
-O0, since we won't be using the definitions for anything anyway. For
lib/System/Path.o when built in Debug+Asserts mode, this leads to a 4%
improvement in compile time (and suppresses 440 function bodies).

<rdar://problem/7987644>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@108156 91177308-0d34-0410-b5e6-96231b3b80d8