Fix broken 2-stage build when -tblgen tools may depend on runpath that is not yet built

[arcivanov]

Summary

In my case -tblgen tools in the second stage end up being built depending on libc++.so.
While the the dependency is correct and the tools are built successfully, -tblgen tools would not function until libc++.so is built in the $ORIGIN/../lib.
But libc++.so of the second stage cannot be built without second stage llvm-tblgen and clang-tblgen functioning.

This patch makes sure that in a bootstrapped build llvm-tblgenand clang-tblgen are always used from the bootstrap even when not crosscompiling.

See https://github.com/llvm/llvm-project/issues/53561 for extensive logs, printouts and cmake caches.

If accepted please also send to 13.x and 14.x.

[arcivanov]

So this change is insufficient as additional tools (lldb-tblgen and clang-ast-dump) will fail thereafter.
Since all those tools depend on libc++ being available for stage2-bins, the libc++ needs to be built on stage two (along with unwind and compiler-rt in my case).
The way I've tried to fix above is treating the symptom and not the cause.

clang/CMakeLists.txt

if (CLANG_ENABLE_BOOTSTRAP)
   endif()
 
   set(COMPILER_OPTIONS
     -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/${CXX_COMPILER}
     -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/${C_COMPILER}
     -DCMAKE_ASM_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/${C_COMPILER}
     -DCMAKE_ASM_COMPILER_ID=Clang)
 
+  # If -tblgen is linked to libc++.so and it's not available in the system, the tool can't run
+  # since the runpath doesn't yet contain libc++.so, which can't be built without -tblgen
+  # See https://github.com/llvm/llvm-project/issues/53561
+  set(${CLANG_STAGE}_TABLEGEN
+    -DLLVM_TABLEGEN=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-tblgen
+    -DCLANG_TABLEGEN=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-tblgen)
+
   # cmake requires CMAKE_LINKER to be specified if the compiler is MSVC-like,
   # otherwise it defaults the linker to be link.exe.
   if(BOOTSTRAP_LLVM_ENABLE_LLD)
     if((WIN32 AND NOT BOOTSTRAP_CMAKE_SYSTEM_NAME) OR BOOTSTRAP_CMAKE_SYSTEM_NAME STREQUAL "Windows")
       set(${CLANG_STAGE}_LINKER -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link${CMAKE_EXECUTABLE_SUFFIX})
     endif()
   endif()
 
   if(BOOTSTRAP_CMAKE_SYSTEM_NAME)
     set(${CLANG_STAGE}_CONFIG -DLLVM_CONFIG_PATH=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-config)
-    set(${CLANG_STAGE}_TABLEGEN
-      -DLLVM_TABLEGEN=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-tblgen
-      -DCLANG_TABLEGEN=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-tblgen)
     if(BOOTSTRAP_CMAKE_SYSTEM_NAME STREQUAL "Linux")
       if(BOOTSTRAP_LLVM_ENABLE_LLD)
         set(${CLANG_STAGE}_LINKER -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/ld.lld)
       endif()
       if(NOT BOOTSTRAP_LLVM_ENABLE_LTO)
         add_dependencies(clang-bootstrap-deps llvm-ar llvm-ranlib)
         set(${CLANG_STAGE}_AR -DCMAKE_AR=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ar)
         set(${CLANG_STAGE}_RANLIB -DCMAKE_RANLIB=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ranlib)