Reapply: [Driver] Use forward slashes in most linker arguments

Reapply: [Driver] Use forward slashes in most linker arguments

libtool inspects the output of $CC -v to detect what object files and
libraries are linked in by default. When clang is built as a native
windows executable, all paths are formatted with backslashes, and
the backslashes cause each argument to be enclosed in quotes. The
backslashes and quotes break further processing within libtool (which
is implemented in shell script, running in e.g. msys) pretty badly.

Between unix style pathes (that only work in tools that are linked
to the msys runtime, essentially the same as cygwin) and proper windows
style paths (with backslashes, that can easily break shell scripts
and msys environments), the best compromise is to use windows style
paths (starting with e.g. c:) but with forward slashes, which both
msys based tools, shell scripts and native windows executables can
cope with. This incidentally turns out to be the form of paths that
GCC prints out when run with -v on windows as well.

This change potentially makes the output from clang -v a bit more
inconsistent, but it is isn't necessarily very consistent to begin with.

Compared to the previous attempt in SVN r345004, this now does
the same transformation on more paths, hopefully on the right set
of paths so that all tests pass (previously some tests failed, where
path fragments that were required to be identical turned out to
use different path separators in different places). This now also
is done only for non-windows, or cygwin/mingw targets, to preserve
all backslashes for MSVC cases (where the paths can end up e.g. embedded
into PDB files. (The transformation function itself,
llvm::sys::path::convert_to_slash only has an effect when run on windows.)

Differential Revision: https://reviews.llvm.org/D53066

llvm-svn: 345370