[pseudo] Add PSEUDO_GEN cmake cache variable to avoid nested CMake invocation

[sammccall]

Summary

Similar to LLVM_TABLEGEN, CLANG_TABLEGEN variables

[mstorsjo]

Looks reasonable to me. The variable name feels a bit un-namespaced though, but that's what the tool is called currently. Should we rename the tool and the variable to give it a bit more of a namespace prefix, like clang-pseudo-gen or so?

[sammccall]

In D126717#3548184, @mstorsjo wrote:

Looks reasonable to me. The variable name feels a bit un-namespaced though, but that's what the tool is called currently. Should we rename the tool and the variable to give it a bit more of a namespace prefix, like clang-pseudo-gen or so?

That makes sense. I'll do that first and rename to CLANG_PSEUDO_GEN.

[sammccall]

Renamed to CLANG_PSEUDO_GEN, landing now.

clang-tools-extra/pseudo/include/CMakeLists.txt

 # The cxx.bnf grammar file
 set(cxx_bnf ${CMAKE_CURRENT_SOURCE_DIR}/../lib/cxx.bnf)
 
-if(LLVM_USE_HOST_TOOLS)
+set(PSEUDO_GEN "pseudo-gen" CACHE
+  STRING "Host pseudo-gen executable. Saves building one if cross-compiling.")
+
+if(NOT PSEUDO_GEN STREQUAL "pseudo-gen")
+  set(pseudo_gen ${PSEUDO_GEN})
+  set(pseudo_gen_target ${PSEUDO_GEN})
+elseif(LLVM_USE_HOST_TOOLS)
   build_native_tool(pseudo-gen pseudo_gen)
   set(pseudo_gen_target "${pseudo_gen}")
 else()
   set(pseudo_gen $<TARGET_FILE:pseudo-gen>)
   set(pseudo_gen_target pseudo-gen)
 endif()
 
 # Generate inc files.