[RFC][WIP] Add an llvmlibc Triple environment

[gchatelet]

Summary

It's probably too early but I'd like to get the conversation started around the integration of llvm-libc in LLVM.
For instance, the avaibility of bzero and bcmp can lead to simplification of libcalls (e.g. memset(pr, 0, size) or memcmp(lhs, rhs, size)==0).

[gchatelet]

• Do not reformat to keep the change clear (modulo making the linter happy)

llvm/include/llvm/ADT/Triple.h

enum EnvironmentType {
     Android,
     Musl,
     MuslEABI,
     MuslEABIHF,
     MuslX32,
 
     MSVC,
     Itanium,
+    LLVMLibc,
     Cygnus,
     CoreCLR,
     Simulator, // Simulator variants of other systems, e.g., Apple's iOS
-    MacABI, // Mac Catalyst variant of Apple's iOS deployment target.
+    MacABI,    // Mac Catalyst variant of Apple's iOS deployment target.
     LastEnvironmentType = MacABI
   };
   enum ObjectFormatType {
     UnknownObjectFormat,
 
     COFF,
     ELF,
     GOFF,

llvm/lib/Support/Triple.cpp

StringRef Triple::getEnvironmentTypeName(EnvironmentType Kind) {
   case Cygnus: return "cygnus";
   case EABI: return "eabi";
   case EABIHF: return "eabihf";
   case GNU: return "gnu";
   case GNUABI64: return "gnuabi64";
   case GNUABIN32: return "gnuabin32";
   case GNUEABI: return "gnueabi";
   case GNUEABIHF: return "gnueabihf";
-  case GNUX32: return "gnux32";
-  case GNUILP32: return "gnu_ilp32";
+  case GNUX32:
+    return "gnux32";
+  case GNUILP32:
+    return "gnu_ilp32";
   case Itanium: return "itanium";
-  case MSVC: return "msvc";
-  case MacABI: return "macabi";
+  case LLVMLibc:
+    return "llvmlibc";
+  case MSVC:
+    return "msvc";
+  case MacABI:
+    return "macabi";
   case Musl: return "musl";
   case MuslEABI: return "musleabi";
   case MuslEABIHF: return "musleabihf";
   case MuslX32: return "muslx32";
   case Simulator: return "simulator";
   }
 
   llvm_unreachable("Invalid EnvironmentType!");