[PATCH] Unify handling of PWD (llvm patch)

[Yunzhong Gao]

  On Ubuntu, if I do "export PWD=blahblah", it will change the command prompt to something like:
  gaogaogao at mydesktop:blahblah$
  but if I run "pwd", my working directory is still unchanged. I assume this means that if I do
  "PWD=blahblah clang test.c"
  clang may not find the current working directory? But I did not test it.

http://llvm-reviews.chandlerc.com/D1335