[CodeGen] Introduce DWARF tag for SwiftTail and emit it in CodeGen.

swifttailcc is a new calling convention in LLVM introduced
in https://reviews.llvm.org/D95443. We add a new DWARF tag to capture
this in debuginfo.

Differential Revision: https://reviews.llvm.org/D95704

(cherry-pick to apple/stable/20210107 from not-yet-merged revision)

Author: varungandhi-apple

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -1320,8 +1320,7 @@ static unsigned getDwarfCC(CallingConv CC) {
   case CC_Swift:
     return llvm::dwarf::DW_CC_LLVM_Swift;
   case CC_SwiftAsync:
-    // [FIXME: swiftasynccc] Update to SwiftAsync once LLVM support lands.
-    return llvm::dwarf::DW_CC_LLVM_Swift;
+    return llvm::dwarf::DW_CC_LLVM_SwiftTail;
   case CC_PreserveMost:
     return llvm::dwarf::DW_CC_LLVM_PreserveMost;
   case CC_PreserveAll:

clang/test/CodeGen/debug-info-cc.c

@@ -57,11 +57,10 @@ __attribute__((swiftcall)) int add_swiftcall(int a, int b) {
   return a+b;
 }
 
-// [FIXME: swiftasynccc] Update debuginfo tag to SwiftAsync once LLVM support lands.
 // LINUX: !DISubprogram({{.*}}"add_swiftasynccall", {{.*}}type: ![[FTY:[0-9]+]]
-// LINUX: ![[FTY]] = !DISubroutineType({{.*}}cc: DW_CC_LLVM_Swift,
-__attribute__((swiftasynccall)) int add_swiftasynccall(int a, int b, int c) {
-  return a+b+c;
+// LINUX: ![[FTY]] = !DISubroutineType({{.*}}cc: DW_CC_LLVM_SwiftTail,
+__attribute__((swiftasynccall)) int add_swiftasynccall(int a, int b) {
+  return a+b;
 }
 
 // LINUX: !DISubprogram({{.*}}"add_inteloclbicc", {{.*}}type: ![[FTY:[0-9]+]]

llvm/include/llvm/BinaryFormat/Dwarf.def

@@ -993,6 +993,7 @@ HANDLE_DW_CC(0xc8, LLVM_Swift)
 HANDLE_DW_CC(0xc9, LLVM_PreserveMost)
 HANDLE_DW_CC(0xca, LLVM_PreserveAll)
 HANDLE_DW_CC(0xcb, LLVM_X86RegCall)
+HANDLE_DW_CC(0xcc, LLVM_SwiftTail)
 // From GCC source code (include/dwarf2.h): This DW_CC_ value is not currently
 // generated by any toolchain.  It is used internally to GDB to indicate OpenCL
 // C functions that have been compiled with the IBM XL C for OpenCL compiler and