clang-cl: Expose -no-canonical-prefixes

-no-canonical-prefixes is a weird flag: In gcc, it controls whether realpath()
is called on the path of the driver binary. It's needed to support some
usecases where gcc is symlinked to, see
https://gcc.gnu.org/ml/gcc/2011-01/msg00429.html for some background.

In clang, the resource dir is found relative to the compiler binary, and
without -no-canonical-prefixes that's an absolute path. For clang, the main use
case for -no-canonical-prefixes is to make the -resource-dir path added by the
driver relative instead of absolute. Making it relative seems like the better
default, but since neither clang not gcc have -canonical-prefixes without no-
which makes changing the default tricky, and since some symlink behaviors do
depend on the realpath() call at least for gcc, just expose
-no-canonical-prefixes in clang-cl mode.

Alternatively we could default to no-canonical-prefix-mode for clang-cl since
it's less likely to be used in symlinked scenarios, but since you already need
to about -no-canonical-prefixes for the non-clang-cl bits of your build, not
hooking this of driver mode seems better to me.

https://reviews.llvm.org/D47480

llvm-svn: 333761