GL_EXT_spirv_intrinsics - Port extensions

Add mechanism to use GL_EXT_spirv_intrinsics headers in glslang.
Ported GL_EXT_shader_realtime_clock as an example.

BUILD.bazel

@@ -49,6 +49,11 @@ py_binary(
     srcs = ["build_info.py"],
 )
 
+py_binary(
+    name = "gen_extension_headers",
+    srcs = ["gen_extension_headers.py"],
+)
+
 genrule(
     name = "gen_build_info_h",
     srcs = ["CHANGES.md", "build_info.h.tmpl"],
@@ -58,6 +63,14 @@ genrule(
     tools = [":build_info"],
 )
 
+genrule(
+    name = "gen_extension_headers_h",
+    srcs = ["glslang/ExtensionHeaders", "gen_extension_headers.py"],
+    outs = ["glslang/glsl_intrinsic_header.h"],
+    cmd_bash = "$(location gen_extension_headers) -i  $(location glslang/ExtensionHeaders) -o $(location glslang/glsl_intrinsic_header.h)",
+    tools = [":gen_extension_headers"],
+)
+
 COMMON_COPTS = select({
     "@bazel_tools//src/conditions:windows": [""],
     "//conditions:default": [
@@ -206,6 +219,7 @@ cc_binary(
     srcs = [
         "StandAlone/StandAlone.cpp",
         "StandAlone/Worklist.h",
+        ":glslang/glsl_intrinsic_header.h"
     ],
     copts = COMMON_COPTS,
     deps = [