Port GetMainExecutable over to PathV2.

I will remove the V1 version as soon as I change clang in the next commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184914 91177308-0d34-0410-b5e6-96231b3b80d8
Remove sys::GetMainExecutable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184916 91177308-0d34-0410-b5e6-96231b3b80d8
Fix the build when __APPLE__ is defined.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184917 91177308-0d34-0410-b5e6-96231b3b80d8