Describe why it exists: what does Shaderc add on top?

Say that Shaderc ships in the Android NDK. Point to the AOSP repos
as the authoritative sources of what is really in the NDK.  Also say
that those repos are downstream from the GitHub repos.

SPIRV-Tools has its own target environment definition, and public
shaderc API has its own. A new one is added into Compiler in
shaderc_util, for better isolation.

Now shaderc_util::Compiler's interface accepts target environment,
not glslang messages anymore. All the conversion to glslang messages
are done in the implementation now.

Also together with the C interface change, new overloads for assembling
are added into the C++ interface.

Two optimization levels are added:
* shaderc_optimization_level_zero
* shaderc_optimization_level_size

The first does no optimization, while the latter runs
strip-debug-info and then unify-constant pass.

This is just for compilation; no assembling support yet.

Change "main function" to "entry point"

This is required to absorb the recent merge of the overloaded-400
branch into Glslang master.
See
https://github.com/KhronosGroup/glslang/commit/ab89bbe702e88c01c2522155c54fc722161276db

Previously, the build was invoking python via the undefined
variable PYTHON_EXECUTABLE.

Fixes https://github.com/google/shaderc/issues/238

SPIRV-Tools source file list has changed.

Id tracking has been refactored.

* Desktop shaders compiling to SPIR-V require version 330 or higher.
* A new control for reporting cascading errors is introduced.

Glslang's builtin function table cache is now sensitive
to the Spv vs. non-Spv and Vulkan vs. GL.
That is https://github.com/KhronosGroup/glslang/issues/166 is fixed.
So now we don't have to tear down Glslang's internal state when
the "message rules" change. That's how Vulkan vs. GL is encoded.

The locations of the headers and grammar files have
moved.  Also the names of the grammar files have changed.