Missing location decoration when generating SPIR-V with auto-map-locations by only `glslang::TShader`

This problem does not occur with the stand alone glslangValidator, I guess the reason is glslangValidator always uses a `glslang::TProgram` to generate SPIR-V.

example GLSL code, with file name `test.frag`:

```glsl
#version 310 es
precision highp float;
layout(location=0) out vec4 fragColor;
uniform float x;
void main()
{
fragColor = vec4(x, x, x, 1.0);
}
```

example stand alone C++ code:

```cpp
#include
#include
#include
#include

const TBuiltInResource DefaultTBuiltInResource = {
/* .MaxLights = */ 32,
/* .MaxClipPlanes = */ 6,
/* .MaxTextureUnits = */ 32,
/* .MaxTextureCoords = */ 32,
/* .MaxVertexAttribs = */ 64,
/* .MaxVertexUniformComponents = */ 4096,
/* .MaxVaryingFloats = */ 64,
/* .MaxVertexTextureImageUnits = */ 32,
/* .MaxCombinedTextureImageUnits = */ 80,
/* .MaxTextureImageUnits = */ 32,
/* .MaxFragmentUniformComponents = */ 4096,
/* .MaxDrawBuffers = */ 32,
/* .MaxVertexUniformVectors = */ 128,
/* .MaxVaryingVectors = */ 8,
/* .MaxFragmentUniformVectors = */ 16,
/* .MaxVertexOutputVectors = */ 16,
/* .MaxFragmentInputVectors = */ 15,
/* .MinProgramTexelOffset = */ -8,
/* .MaxProgramTexelOffset = */ 7,
/* .MaxClipDistances = */ 8,
/* .MaxComputeWorkGroupCountX = */ 65535,
/* .MaxComputeWorkGroupCountY = */ 65535,
/* .MaxComputeWorkGroupCountZ = */ 65535,
/* .MaxComputeWorkGroupSizeX = */ 1024,
/* .MaxComputeWorkGroupSizeY = */ 1024,
/* .MaxComputeWorkGroupSizeZ = */ 64,
/* .MaxComputeUniformComponents = */ 1024,
/* .MaxComputeTextureImageUnits = */ 16,
/* .MaxComputeImageUniforms = */ 8,
/* .MaxComputeAtomicCounters = */ 8,
/* .MaxComputeAtomicCounterBuffers = */ 1,
/* .MaxVaryingComponents = */ 60,
/* .MaxVertexOutputComponents = */ 64,
/* .MaxGeometryInputComponents = */ 64,
/* .MaxGeometryOutputComponents = */ 128,
/* .MaxFragmentInputComponents = */ 128,
/* .MaxImageUnits = */ 8,
/* .MaxCombinedImageUnitsAndFragmentOutputs = */ 8,
/* .MaxCombinedShaderOutputResources = */ 8,
/* .MaxImageSamples = */ 0,
/* .MaxVertexImageUniforms = */ 0,
/* .MaxTessControlImageUniforms = */ 0,
/* .MaxTessEvaluationImageUniforms = */ 0,
/* .MaxGeometryImageUniforms = */ 0,
/* .MaxFragmentImageUniforms = */ 8,
/* .MaxCombinedImageUniforms = */ 8,
/* .MaxGeometryTextureImageUnits = */ 16,
/* .MaxGeometryOutputVertices = */ 256,
/* .MaxGeometryTotalOutputComponents = */ 1024,
/* .MaxGeometryUniformComponents = */ 1024,
/* .MaxGeometryVaryingComponents = */ 64,
/* .MaxTessControlInputComponents = */ 128,
/* .MaxTessControlOutputComponents = */ 128,
/* .MaxTessControlTextureImageUnits = */ 16,
/* .MaxTessControlUniformComponents = */ 1024,
/* .MaxTessControlTotalOutputComponents = */ 4096,
/* .MaxTessEvaluationInputComponents = */ 128,
/* .MaxTessEvaluationOutputComponents = */ 128,
/* .MaxTessEvaluationTextureImageUnits = */ 16,
/* .MaxTessEvaluationUniformComponents = */ 1024,
/* .MaxTessPatchComponents = */ 120,
/* .MaxPatchVertices = */ 32,
/* .MaxTessGenLevel = */ 64,
/* .MaxViewports = */ 16,
/* .MaxVertexAtomicCounters = */ 0,
/* .MaxTessControlAtomicCounters = */ 0,
/* .MaxTessEvaluationAtomicCounters = */ 0,
/* .MaxGeometryAtomicCounters = */ 0,
/* .MaxFragmentAtomicCounters = */ 8,
/* .MaxCombinedAtomicCounters = */ 8,
/* .MaxAtomicCounterBindings = */ 1,
/* .MaxVertexAtomicCounterBuffers = */ 0,
/* .MaxTessControlAtomicCounterBuffers = */ 0,
/* .MaxTessEvaluationAtomicCounterBuffers = */ 0,
/* .MaxGeometryAtomicCounterBuffers = */ 0,
/* .MaxFragmentAtomicCounterBuffers = */ 1,
/* .MaxCombinedAtomicCounterBuffers = */ 1,
/* .MaxAtomicCounterBufferSize = */ 16384,
/* .MaxTransformFeedbackBuffers = */ 4,
/* .MaxTransformFeedbackInterleavedComponents = */ 64,
/* .MaxCullDistances = */ 8,
/* .MaxCombinedClipAndCullDistances = */ 8,
/* .MaxSamples = */ 4,
/* .maxMeshOutputVerticesNV = */ 256,
/* .maxMeshOutputPrimitivesNV = */ 512,
/* .maxMeshWorkGroupSizeX_NV = */ 32,
/* .maxMeshWorkGroupSizeY_NV = */ 1,
/* .maxMeshWorkGroupSizeZ_NV = */ 1,
/* .maxTaskWorkGroupSizeX_NV = */ 32,
/* .maxTaskWorkGroupSizeY_NV = */ 1,
/* .maxTaskWorkGroupSizeZ_NV = */ 1,
/* .maxMeshViewCountNV = */ 4,
/* .maxDualSourceDrawBuffersEXT = */ 1,

/* .limits = */ {
/* .nonInductiveForLoops = */ 1,
/* .whileLoops = */ 1,
/* .doWhileLoops = */ 1,
/* .generalUniformIndexing = */ 1,
/* .generalAttributeMatrixVectorIndexing = */ 1,
/* .generalVaryingIndexing = */ 1,
/* .generalSamplerIndexing = */ 1,
/* .generalVariableIndexing = */ 1,
/* .generalConstantMatrixVectorIndexing = */ 1,
}
};

std::string readFile(const char* filename) {
std::ifstream fin(filename);
if (fin.fail())
{
printf("Open %s faild\n", filename);
exit(1);
}
fin.seekg(0, std::ios::end);
size_t size = fin.tellg();
std::string buffer(size, '\0');
fin.seekg(0);
fin.read(&buffer[0], size);
return buffer;
}

int main(int argc, char** argv)
{
if (argc < 2)
{
printf("usage: %s \n", argv[0]);
return 0;
}

glslang::InitializeProcess();
std::string source = readFile(argv[1]);
const char* srcs[] = { source.c_str() };
glslang::TShader shader(EShLangFragment);
shader.setStrings(srcs, 1);
shader.setEnvInput(glslang::EShSourceGlsl, EShLangFragment, glslang::EShClientOpenGL, 310);
shader.setEnvClient(glslang::EShClientOpenGL, glslang::EShTargetOpenGL_450);
shader.setEnvTarget(glslang::EShTargetSpv, glslang::EShTargetSpv_1_0);

shader.setAutoMapLocations(true);
shader.addUniformLocationOverride("x", 2);

if (!shader.parse(&DefaultTBuiltInResource, 310, true, EShMsgDefault))
{
printf("Error: %s", shader.getInfoLog());
exit(1);
}

std::vector spirv;
glslang::SpvOptions opt;
glslang::GlslangToSpv(*shader.getIntermediate(), spirv, &opt);
glslang::OutputSpvBin(spirv, "frag.spv");
}
```

After compile the code to `test`, running `./test test.frag`, the generated `frag.spv` disassembled:
```spirv
; SPIR-V
; Version: 1.0
; Generator: Khronos Glslang Reference Front End; 10
; Bound: 17
; Schema: 0
OpCapability Shader
%1 = OpExtInstImport "GLSL.std.450"
OpMemoryModel Logical GLSL450
OpEntryPoint Fragment %main "main" %fragColor
OpExecutionMode %main OriginLowerLeft
OpSource ESSL 310
OpName %main "main"
OpName %fragColor "fragColor"
OpName %x "x"
OpDecorate %fragColor Location 0
%void = OpTypeVoid
%3 = OpTypeFunction %void
%float = OpTypeFloat 32
%v4float = OpTypeVector %float 4
%_ptr_Output_v4float = OpTypePointer Output %v4float
%fragColor = OpVariable %_ptr_Output_v4float Output
%_ptr_UniformConstant_float = OpTypePointer UniformConstant %float
%x = OpVariable %_ptr_UniformConstant_float UniformConstant
%float_1 = OpConstant %float 1
%main = OpFunction %void None %3
%5 = OpLabel
%12 = OpLoad %float %x
%13 = OpLoad %float %x
%14 = OpLoad %float %x
%16 = OpCompositeConstruct %v4float %12 %13 %14 %float_1
OpStore %fragColor %16
OpReturn
OpFunctionEnd
```

The difference with run `glslangValidator --auto-map-locations -ux:2 -G test.frag` is, there are no two `OpDecorate` instructions:

```spirv
OpDecorate %x Location 2
OpDecorate %x DescriptorSet 0
```