[LLVM] Add environment triple for 'llvm' (#117218)

Summary:
The LLVM C library is an in-development environment for running
executables on various systems. Similarly how we have `-gnu` to indicate
that we are using a GNU toolchain we should support `-llvm` to indicate
the LLVM C library. This patch only adds the basic support for the
triple and does not do any necessary clang changes to handle compiling
with it.

Fixes #117251

Author: jhuber6

llvm/include/llvm/TargetParser/Triple.h

@@ -268,6 +268,7 @@ class Triple {
     MuslF32,
     MuslSF,
     MuslX32,
+    LLVM,
 
     MSVC,
     Itanium,

llvm/lib/TargetParser/Triple.cpp

@@ -360,6 +360,8 @@ StringRef Triple::getEnvironmentTypeName(EnvironmentType Kind) {
   case OpenHOS: return "ohos";
   case PAuthTest:
     return "pauthtest";
+  case LLVM:
+    return "llvm";
   }
 
   llvm_unreachable("Invalid EnvironmentType!");
@@ -740,6 +742,7 @@ static Triple::EnvironmentType parseEnvironment(StringRef EnvironmentName) {
       .StartsWith("opencl", Triple::OpenCL)
       .StartsWith("ohos", Triple::OpenHOS)
       .StartsWith("pauthtest", Triple::PAuthTest)
+      .StartsWith("llvm", Triple::LLVM)
       .Default(Triple::UnknownEnvironment);
 }
 

llvm/unittests/TargetParser/TripleTest.cpp

@@ -1341,6 +1341,12 @@ TEST(TripleTest, ParsedIDs) {
   EXPECT_TRUE(T.isTime64ABI());
   EXPECT_TRUE(T.isHardFloatABI());
 
+  T = Triple("x86_64-pc-linux-llvm");
+  EXPECT_EQ(Triple::x86_64, T.getArch());
+  EXPECT_EQ(Triple::PC, T.getVendor());
+  EXPECT_EQ(Triple::Linux, T.getOS());
+  EXPECT_EQ(Triple::LLVM, T.getEnvironment());
+
   T = Triple("huh");
   EXPECT_EQ(Triple::UnknownArch, T.getArch());
 }