[lldb] skip host build for lldb_tblgen with LLDB_TABLEGEN_EXE set

[manojgupta]

Summary

When cross compiling lldb-server, do not create a host build
for building lldb-tblgeb when LLDB_TABLEGEN_EXE is already
provided. This avoids an expensive and time-consuming build step
if lldb-tblgen was already built previously for host.

[manojgupta]

This is a local patch we have been carrying in Chrome OS.
What we do to cross compile lldb-server:

1. Create host build to build tools like llvm-tblgen, clang-tblgen, lldb-tblgen
2. Cross compile llvm and clang libraries using the just build *tblgen.
3. Cross compile lldb by setting the path to pre-cross compiled libraries and *tblgen binaries from step 1.

Without this patch, cross compiling lldb-server start yet another host build just to build lldb-tblgen which we want to avoid.
Also see https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2851114/12/dev-util/lldb-server/lldb-server-12.0_pre416183.ebuild

[JDevlieghere]

Looks reasonable to me.

lldb/CMakeLists.txt

 # any source file in LLDB as the imported Clang modules might include
 # some of these generated headers. This approach is copied from Clang's main
 # CMakeLists.txt, so it should kept in sync the code in Clang which was added
 # in llvm-svn 308844.
 if(LLVM_ENABLE_MODULES)
   list(APPEND LLVM_COMMON_DEPENDS intrinsics_gen)
 endif()
 
-if(CMAKE_CROSSCOMPILING AND LLDB_BUILT_STANDALONE)
+if(CMAKE_CROSSCOMPILING AND LLDB_BUILT_STANDALONE AND NOT LLDB_TABLEGEN_EXE)
   set(LLVM_USE_HOST_TOOLS ON)
   include(CrossCompile)
   if (NOT NATIVE_LLVM_DIR OR NOT NATIVE_Clang_DIR)
     message(FATAL_ERROR
       "Crosscompiling standalone requires the variables NATIVE_{CLANG,LLVM}_DIR
       for building the native lldb-tblgen used during the build process.")
   endif()
   llvm_create_cross_target(lldb NATIVE "" Release