Generate build information from CHANGES.md

This PR significantly reworks the way glslang is versioned.

Instead of committing changes to the `GLSLANG_MINOR_VERSION` define in
`glslang/Public/ShaderLang.h`, and using `make-revision` to generate
`GLSLANG_PATCH_LEVEL` in `glslang/Include/revision.h`, all version
information is now derived from the new `CHANGES.md` file.

`CHANGES.md` acts as the single source of truth for glslang version
information, along with a convenient place to put all release notes for
each notable change made.

`CHANGES.md` is parsed using the new `build_info.py` python script.
This script can read basic template files to produce new source files,
which it does to read the new `build_info.h.tmpl` to generate (at build
time) a glslang private header at
`<build-dir>/include/glslang/build_info.h`.
I've written generators for each of the CMake, Bazel, gn, and
`Android.mk` build scripts.

The new version code conforms to the Semantic Versioning 2.0 spec.

This new version is also used by the CMake rules to produce versioned
shared objects, including a major-versioned SONAME.

New APIs:
---------

* `glslang::GetVersion()` returns a `Version` struct with the version
  major, minor, patch and flavor.

Breaking API changes:
---------------------

* The public defines `GLSLANG_MINOR_VERSION` and `GLSLANG_PATCH_LEVEL`
  have been entirely removed.
* `glslang/Public/ShaderLang.h` and `glslang/Include/revision.h` have
  been deleted.
* Instead, `<build-dir>/include/glslang/build_info.h` is created in
  the build directory, and `<build-dir>/include` is a CMake `PUBLIC`
  (dependee-inherited) include directory for the glslang targets.
* `<build-dir>/include/glslang/build_info.h` contains the following
   new #defines:
   `GLSLANG_VERSION_MAJOR`, `GLSLANG_VERSION_MINOR`,
   `GLSLANG_VERSION_PATCH`, `GLSLANG_VERSION_FLAVOR`,
   `GLSLANG_VERSION_GREATER_THAN(major, minor, patch)`,
   `GLSLANG_VERSION_GREATER_OR_EQUAL_TO(major, minor, patch)`,
   `GLSLANG_VERSION_LESS_THAN(major, minor, patch)`,
   `GLSLANG_VERSION_LESS_OR_EQUAL_TO(major, minor, patch)`
*  The CMake install output directory contains a copy of
   `build_info.h` at: `include/glslang/build_info.h`
*  Python3 is now always required to build glslang (likely always
   required for transitive dependency builds).

BUILD.gn

@@ -46,6 +46,29 @@ if (defined(is_fuchsia_tree) && is_fuchsia_tree) {
   _configs_to_add = [ "//build/config/compiler:no_chromium_code" ]
 }
 
+action("glslang_build_info") {
+  script = "build_info.py"
+
+  src_dir = "."
+  changes_file = "CHANGES.md"
+  template_file = "build_info.h.tmpl"
+  out_file = "${target_gen_dir}/include/glslang/build_info.h"
+
+  inputs = [
+    changes_file,
+    script,
+    template_file,
+  ]
+  outputs = [
+    out_file
+  ]
+  args = [
+    rebase_path(src_dir, root_build_dir),
+    "-i", rebase_path(template_file, root_build_dir),
+    "-o", rebase_path(out_file, root_build_dir),
+  ]
+}
+
 spirv_tools_dir = glslang_spirv_tools_dir
 if (!defined(glslang_angle)) {
   glslang_angle = false
@@ -109,7 +132,6 @@ template("glslang_sources_common") {
       "glslang/Include/Types.h",
       "glslang/Include/arrays.h",
       "glslang/Include/intermediate.h",
-      "glslang/Include/revision.h",
       "glslang/MachineIndependent/Constant.cpp",
       "glslang/MachineIndependent/InfoSink.cpp",
       "glslang/MachineIndependent/Initialize.cpp",
@@ -216,13 +238,18 @@ template("glslang_sources_common") {
       ]
     }
 
+    deps = [ ":glslang_build_info" ]
+
     if (invoker.enable_opt) {
-      deps = [
+      deps += [
+        ":glslang_build_info",
         "${spirv_tools_dir}:spvtools_opt",
         "${spirv_tools_dir}:spvtools_val",
       ]
     }
 
+    include_dirs = [ "${target_gen_dir}/include" ]
+
     configs -= _configs_to_remove
     configs += _configs_to_add
   }
@@ -265,6 +292,8 @@ executable("glslang_validator") {
   ]
   public_configs = [ ":glslang_hlsl" ]
 
+  include_dirs = [ "${target_gen_dir}/include" ]
+
   configs -= _configs_to_remove
   configs += _configs_to_add
 }