Re-land "[Driver] Support default libc++ library location on Darwin"

This reverts commit 05eeed9691aeb3e0316712195b998e9078cdceb0 and after
fixing the impacted lldb tests in 5d1c43f333c2327be61604dc90ea675f0d1e6913.

  [Driver] Support default libc++ library location on Darwin

  Darwin driver currently uses libc++ headers that are part of Clang
  toolchain when available (by default ../include/c++/v1 relative to
  executable), but it completely ignores the libc++ library itself
  because it doesn't pass the location of libc++ library that's part
  of Clang (by default ../lib relative to the exceutable) to the linker
  always using the system copy of libc++.

  This may lead to subtle issues when the compilation fails because the
  headers that are part of Clang toolchain are incompatible with the
  system library. Either the driver should ignore both headers as well as
  the library, or it should always try to use both when available.

  This patch changes the driver behavior to do the latter which seems more
  reasonable, it makes it easy to test and use custom libc++ build on
  Darwin while still allowing the use of system version. This also matches
  the Clang driver behavior on other systems.

  Differential Revision: https://reviews.llvm.org/D45639

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