Only use argv[0] as the main executable name if it exists.

Only use argv[0] as the main executable name if it exists.

Under some environments, argv[0] doesn't hold a valid file name, but
sys::fs::getMainExecutable will find the main executable properly.

This patch tweaks the logic to fall back to sys::fs::getMainExecutable
in more situations.

Differential Revision: https://reviews.llvm.org/D60730

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358455 91177308-0d34-0410-b5e6-96231b3b80d8