Escape command line arguments in backtraces

A common routine is to have the compiler crash, and attempt to rerun the
cc1 command-line by copying and pasting the arguments printed by
`llvm::Support::PrettyStackProgram::print`. However, these arguments are
not quoted or escaped which means they must be manually edited before
working correctly. This patch ensures that shell-unfriendly characters
are C-escaped, and arguments with spaces are double-quoted reducing the
frustration of running cc1 inside a debugger.

As the quoting is C, this is "best effort for most shells", but should
be fine for at least bash, zsh, csh, and cmd.exe.

Reviewed by: jhenderson

Differential Revision: https://reviews.llvm.org/D90759