[PATCH] [libcxx] Teach libcxx's lit configuration new ways to find lit.site.cfg

[Jonathan Roelofs]

Is this compatible with `--config-prefix=`?

================
Comment at: test/lit.site.cfg:1
@@ +1,2 @@
+# -*- Python -*- vim: set syntax=python tabstop=4 expandtab cc=80:
+
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We don't usually put the vim part of this in llvm things, do we?

http://reviews.llvm.org/D6255