[llvm][CMake] Improve error message for unknown or experimental targets

Previously you would get this error when passing an experimental target
via LLVM_TARGETS_TO_BUILD:
```
cmake ../llvm-project/llvm -DLLVM_TARGETS_TO_BUILD=M68k -DCMAKE_BU
ILD_TYPE=Release -GNinja

CMake Error at CMakeLists.txt:929 (message):
  The target `M68k' is experimental and must be passed via
  LLVM_EXPERIMENTAL_TARGETS_TO_BUILD
```

Since M68k is a known experimental target, this is helpful. However,
any made up target name would give you the same error.
```
cmake ../llvm-project/llvm -DLLVM_TARGETS_TO_BUILD=NotATarget -DCMAKE_BUILD_TYPE=Release -GNinja

CMake Error at CMakeLists.txt:929 (message):
  The target `NotATarget' is experimental and must be passed via
  LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.
```

We know the set of default targets and in tree experimental targets,
so let's be more specific if we know that it is not an in tree experimental
target:
```
CMake Error at CMakeLists.txt:934 (message):
  The target `NotATarget' is not a default target.  It may be experimental,
  if so it must be passed via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.

  Core tier targets:
  AArch64;AMDGPU;ARM;AVR;BPF;Hexagon;Lanai;LoongArch;Mips;MSP430;NVPTX;PowerPC;RISCV;Sparc;SystemZ;VE;WebAssembly;X86;XCore

  Known experimental targets: ARC;CSKY;DirectX;M68k;SPIRV;Xtensa
```

It "may" be an experimental target because we do allow users to specify
targets that are not in LLVM_ALL_EXPERIMENTAL_TARGETS, and they will work
as long as lib/Target/<target> exists.

Maybe that could be made more strict but it would break a bunch of
forks for not much gain.

The known target names are listed to help users trying to configure
architectures with confusing naming schemes, for example Arm. Which is
variously called ARM/Arm/Armv7/AArch32 across llvm and other software.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D157844

Author: DavidSpickett

llvm/CMakeLists.txt

@@ -919,15 +919,21 @@ set(LLVM_ENUM_EXEGESIS "")
 foreach(t ${LLVM_TARGETS_TO_BUILD})
   set( td ${LLVM_MAIN_SRC_DIR}/lib/Target/${t} )
 
-  list(FIND LLVM_ALL_TARGETS ${t} idx)
-  list(FIND LLVM_EXPERIMENTAL_TARGETS_TO_BUILD ${t} idy)
-  # At this point, LLVMBUILDTOOL already checked all the targets passed in
-  # LLVM_TARGETS_TO_BUILD and LLVM_EXPERIMENTAL_TARGETS_TO_BUILD, so
-  # this test just makes sure that any experimental targets were passed via
+  # Make sure that any experimental targets were passed via
   # LLVM_EXPERIMENTAL_TARGETS_TO_BUILD, not LLVM_TARGETS_TO_BUILD.
-  if( idx LESS 0 AND idy LESS 0 )
-    message(FATAL_ERROR "The target `${t}' is experimental and must be passed "
-      "via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.")
+  # We allow experimental targets that are not in LLVM_ALL_EXPERIMENTAL_TARGETS,
+  # as long as they are passed via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.
+  if ( NOT ${t} IN_LIST LLVM_ALL_TARGETS AND NOT ${t} IN_LIST LLVM_EXPERIMENTAL_TARGETS_TO_BUILD )
+    if( ${t} IN_LIST LLVM_ALL_EXPERIMENTAL_TARGETS )
+      message(FATAL_ERROR "The target `${t}' is experimental and must be passed "
+        "via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.")
+    else()
+      message(FATAL_ERROR "The target `${t}' is not a core tier target. It may be "
+        "experimental, if so it must be passed via "
+        "LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.\n"
+        "Core tier targets: ${LLVM_ALL_TARGETS}\n"
+        "Known experimental targets: ${LLVM_ALL_EXPERIMENTAL_TARGETS}")
+    endif()
   else()
     set(LLVM_ENUM_TARGETS "${LLVM_ENUM_TARGETS}LLVM_TARGET(${t})\n")
   endif()