[PATCH] [libcxx] Refactor CMakeLists.txt handling of compile and link flags to suppress warnings.

[Eric Fiselier]

Hi danalbert, mclow.lists,

Currently we have 5 variables that are used to specify options for building libcxx
1. `LIBCXX_CXX_FEATURE_FLAGS`
2. `LIBCXX_CXX_WARNING_FLAGS`
3. `LIBCXX_CXX_REQUIRED_FLAGS`
4. `compile_flags` (in libcxx/lib)
5. `link_flags` (in libcxx/lib)

The first three all get put into `CMAKE_CXX_FLAGS`. 
This changes the way flags are handled by only using 3 different options:

1. `LIBCXX_CXX_FLAGS` - general compile and link flags.
2. `LIBCXX_COMPILE_FLAGS` - compile only flags.
3. `LIBCXX_LINK_FLAGS` - link only flags.

This patch also removes the warning about `-nostdinc++` being unused during linking.

http://reviews.llvm.org/D6277

Files:
  CMakeLists.txt
  lib/CMakeLists.txt
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