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.bazel

@@ -43,6 +43,20 @@ licenses(["notice"])
 
 exports_files(["LICENSE"])
 
+# Build information generation script
+py_binary(
+    name = "build_info",
+    srcs = ["build_info.py"],
+)
+
+genrule(
+    name = "gen_build_info_h",
+    srcs = ["CHANGES.md", "build_info.h.tmpl"],
+    outs = ["glslang/build_info.h"],
+    cmd = "$(location build_info) $$(dirname $(location CHANGES.md)) -i $(location build_info.h.tmpl) -o $(location glslang/build_info.h)",
+    tools = [":build_info"],
+)
+
 COMMON_COPTS = select({
     "@bazel_tools//src/conditions:windows": [""],
     "//conditions:default": [
@@ -93,6 +107,7 @@ cc_library(
         "StandAlone/DirStackFileIncluder.h",
         "glslang/OSDependent/osinclude.h",
         "glslang/Public/ShaderLang.h",
+        ":gen_build_info_h",
     ],
     copts = COMMON_COPTS,
     defines = [