Teach Lit to catch OSError exceptions when creating a process during the
execution of a shell command. This can happen for example if the
``RUN:`` line calls a python script which can work correctly under
Linux/OSX but will not work under Windows. A more useful error message
is now shown rather than an unhelpful backtrace.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220227 91177308-0d34-0410-b5e6-96231b3b80d8
Constify input argument of RelocVisitor and DWARFContext constructors. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220228 91177308-0d34-0410-b5e6-96231b3b80d8
Be more specific about return type of MachOUniversalBinary::getObjectForArch

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220230 91177308-0d34-0410-b5e6-96231b3b80d8
Add const version of OwningBinary::getBinary

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220231 91177308-0d34-0410-b5e6-96231b3b80d8
Constify getELFDynamicSymbolIterators standalone function. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220232 91177308-0d34-0410-b5e6-96231b3b80d8
Try to fix GCC error about invalid use of const_cast in const version of ErrorOr::get()

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220233 91177308-0d34-0410-b5e6-96231b3b80d8