Add support for DIStringType in DWARF

Add support for `DIStringType` in DWARF. This is Fortran
specific case (Fortran `CHARACTER` type). The `CHARACTER` type
is mapped to the DWARF tag `DW_TAG_string_type`
from `DIStringType` from LLVM metadata, e.g.:

LLVM Metadata
```
!0 = !DIStringType(name: "character(10)", size: 80, align: 8)
```

DWARF information
```
DW_TAG_string_type
  DW_AT_name: character(10)
  DW_AT_byte_size: 10
```

Author: matborzyszkowski

IGC/DebugInfo/DwarfCompileUnit.cpp

@@ -695,6 +695,10 @@ IGC::DIE *CompileUnit::getOrCreateTypeDIE(const MDNode *TyNode) {
 
   if (isa<DIBasicType>(Ty))
     constructTypeDIE(*TyDIE, cast<DIBasicType>(Ty));
+#if LLVM_VERSION_MAJOR >= 12
+  else if (isa<DIStringType>(Ty))
+    constructTypeDIE(*TyDIE, cast<DIStringType>(Ty));
+#endif
   else if (isa<DICompositeType>(Ty))
     constructTypeDIE(*TyDIE, cast<DICompositeType>(Ty));
   else if (isa<DISubroutineType>(Ty))
@@ -1432,6 +1436,43 @@ void CompileUnit::constructTypeDIE(DIE &Buffer, DIBasicType *BTy) {
   addUInt(&Buffer, dwarf::DW_AT_byte_size, None, Size);
 }
 
+#if LLVM_VERSION_MAJOR >= 12
+/// constructTypeDIE - Construct basic type die from DIStringType.
+void CompileUnit::constructTypeDIE(DIE &Buffer, DIStringType *STy) {
+  // Get core information.
+  StringRef Name = STy->getName();
+  // Add name if not anonymous or intermediate type.
+  if (!Name.empty())
+    addString(&Buffer, dwarf::DW_AT_name, Name);
+
+  if (DIVariable *Var = STy->getStringLength()) {
+    if (auto *VarDIE = getDIE(Var))
+      addDIEEntry(&Buffer, dwarf::DW_AT_string_length, VarDIE);
+  } else if (DIExpression *Expr = STy->getStringLengthExp()) {
+    DIEBlock *Loc = new (DIEValueAllocator) DIEBlock;
+    DIEDwarfExpression DwarfExpr(*Asm, getCU(), *Loc);
+    DwarfExpr.addExpression(Expr);
+    addBlock(&Buffer, dwarf::DW_AT_string_length, DwarfExpr.finalize());
+  } else {
+    uint64_t Size = STy->getSizeInBits() >> 3;
+    addUInt(&Buffer, dwarf::DW_AT_byte_size, None, Size);
+  }
+
+  if (DIExpression *Expr = STy->getStringLocationExp()) {
+    DIEBlock *Loc = new (DIEValueAllocator) DIEBlock;
+    DIEDwarfExpression DwarfExpr(*Asm, getCU(), *Loc);
+    DwarfExpr.addExpression(Expr);
+    addBlock(&Buffer, dwarf::DW_AT_data_location, DwarfExpr.finalize());
+  }
+
+  if (STy->getEncoding()) {
+    // For eventual Unicode support.
+    addUInt(&Buffer, dwarf::DW_AT_encoding, dwarf::DW_FORM_data1,
+            STy->getEncoding());
+  }
+}
+#endif
+
 /// constructTypeDIE - Construct derived type die from DIDerivedType.
 void CompileUnit::constructTypeDIE(DIE &Buffer, DIDerivedType *DTy) {
   // Get core information.

IGC/DebugInfo/DwarfCompileUnit.hpp

@@ -442,6 +442,11 @@ class CompileUnit {
   /// constructTypeDIE - Construct derived type die from llvm::DIDerivedType.
   void constructTypeDIE(DIE &Buffer, llvm::DIDerivedType *DTy);
 
+#if LLVM_VERSION_MAJOR >= 12
+  /// constructTypeDIE - Construct derived type die from llvm::DIStringType.
+  void constructTypeDIE(DIE &Buffer, llvm::DIStringType *DTy);
+#endif
+
   /// constructTypeDIE - Construct type DIE from DICompositeType.
   void constructTypeDIE(DIE &Buffer, llvm::DICompositeType *CTy);