Fix a tiny bug in -no-canonical-prefixes that somehow we have never

noticed until now.

The code for setting up the driver's InstalledDir didn't respect
-no-canonical-prefixes. Because of this, there are a few places in the
driver where we would unexpectedly form absolute paths, notably when
searching for and finding GCC installations to use, etc. The fix is
straightforward, and I've added this path to '-v' both so we can test it
sanely and so that it will be substantially more obvious the next time
someone has to debug something here.

Note that there is another bug that we don't actually *canonicalize* the
installed directory! I don't really want to fix that because I don't
have a realistic way to test the usage of this mode. I suspect that
folks using the shared module cache would care about getting this right
though, and so they might want to address it. I've left the appropriate
FIXMEs so that it is clear what to change, and I've updated the test
code to make it clear what is happening here.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244065 91177308-0d34-0410-b5e6-96231b3b80d8