llpc/dbginfo: streamline flags in DebugFunction

DebugFunction is always a function definition, never a function
declaration -- those are represented using DebugFunctionDeclaration.

It's also unclear whether shader entry points should be considered "main
subprograms" or not. First of all, it's unclear what that flag means.
Second, entry points aren't exactly like C/C++ main() -- a better
analogy would be the "main" function of a thread created with
pthread_create or similar. Finally, the current test is simply incorrect
because NonSemantic.Shader.DebugInfo.100 doesn't actually reference the
OpFunction from the DebugFunction instruction.

llpc/translator/lib/SPIRV/SPIRVToLLVMDbgTran.cpp

@@ -477,15 +477,11 @@ DINode *SPIRVToLLVMDbgTran::transFunction(const SPIRVExtInst *DebugInst) {
 
   SPIRVWord SPIRVDebugFlags = getConstant(Ops[FlagsIdx]);
   DINode::DIFlags Flags = mapToDIFlags(SPIRVDebugFlags);
-  // TODO: IsDefinition is always true for DebugFunction, but should be false
-  // for DebugFunctionDeclaration.
-  const bool IsDefinition = true;
-
   bool IsOptimized = SPIRVDebugFlags & SPIRVDebug::FlagIsOptimized;
   bool IsLocal = SPIRVDebugFlags & SPIRVDebug::FlagIsLocal;
-  bool IsMainSubprogram = BM->getEntryPoint(Ops[FunctionIdIdx]) != nullptr;
   DISubprogram::DISPFlags SPFlags =
-      DISubprogram::toSPFlags(IsLocal, IsDefinition, IsOptimized, DISubprogram::SPFlagNonvirtual, IsMainSubprogram);
+      DISubprogram::toSPFlags(IsLocal, /*IsDefinition*/ true, IsOptimized, DISubprogram::SPFlagNonvirtual,
+                              /*IsMainSubprogram*/ false);
 
   unsigned ScopeLine = getConstant(Ops[ScopeLineIdx]);