[lumped builds] Add include guards (#pragma once) to header files that did not have any.

The codebase seems to use both “#pragma once“ approach, and “#ifndef / #define” approach,
so I picked pragma once as that one is less typing & less brittle.

When glslang is built with some other build system and lumped/unity builds are used,
without the include guards some headers would get included multiple times, leading to duplicate
declaration errors.

SPIRV/GlslangToSpv.h

@@ -32,6 +32,8 @@
 // ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 // POSSIBILITY OF SUCH DAMAGE.
 
+#pragma once
+
 #if _MSC_VER >= 1900
     #pragma warning(disable : 4464) // relative include path contains '..'
 #endif

SPIRV/doc.h

@@ -36,6 +36,8 @@
 // Parameterize the SPIR-V enumerants.
 //
 
+#pragma once
+
 #include "spirv.hpp"
 
 #include <vector>

glslang/MachineIndependent/LiveTraverser.h

@@ -33,6 +33,8 @@
 // POSSIBILITY OF SUCH DAMAGE.
 //
 
+#pragma once
+
 #include "../Include/Common.h"
 #include "reflection.h"
 #include "localintermediate.h"

glslang/MachineIndependent/RemoveTree.h

@@ -32,6 +32,8 @@
 // POSSIBILITY OF SUCH DAMAGE.
 //
 
+#pragma once
+
 namespace glslang {
 
 void RemoveAllTreeNodes(TIntermNode*);

glslang/MachineIndependent/ScanContext.h

@@ -38,6 +38,8 @@
 // sits between the preprocessor scanner and parser.
 //
 
+#pragma once
+
 #include "ParseHelper.h"
 
 namespace glslang {

glslang/MachineIndependent/gl_types.h

@@ -21,6 +21,8 @@
 ** MATERIALS OR THE USE OR OTHER DEALINGS IN THE MATERIALS.
 */
 
+#pragma once
+
 #define GL_FLOAT                          0x1406
 #define GL_FLOAT_VEC2                     0x8B50
 #define GL_FLOAT_VEC3                     0x8B51

glslang/MachineIndependent/propagateNoContraction.h

@@ -37,6 +37,8 @@
 // propagate 'noContraction' qualifier.
 //
 
+#pragma once
+
 #include "../Include/intermediate.h"
 
 namespace glslang {