[PATCH] [libcxx] Refactor config.py internals to more easily allow for custom compiler configurations.

[Eric Fiselier]

Hi danalbert, jroelofs,

This patch refactors the structure for configuring all compiler flags to make it easier to run the test suite against non-standard configurations. It does so using the following LIT options:
1. `--param=no_default_flags`: Disable ALL flags other than those explicitly specified with `--param=compile_flags` and `--param=link_flags`.
2. `--param=no_library_flags`: Disable only the flags related to configuring the test suite for the just built libc++. This disables adding the include paths to the libc++ headers as well as the link flags relating to linking libc++ and the abi library.

The rational for these changes is:
1. To fix a bug where `no_default_flags` currently doesn't disable *all* flags.
2. To allow a user to run the test suite without having to manually define a bunch of flags that are always required (ex `-Ipath/to/libcxx/test/support`)

An example use case for these changes is running the test-suite against libstdc++. Previously you would have to use an invocation that looked something like:
```
lit -sv --param=no_default_flags=True --param=compile_flags='-I/path/to/libcxx/test/support -std=c++11' path/to/tests
```
Now you can simply use:
lit -sv --param=no_library_flags=True path/to/tests

This patch is useful to me when running the benchmarks against libstdc++.

http://reviews.llvm.org/D8459

Files:
  test/libcxx/test/config.py

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