[lldb] Add EXPORT to lldb-tblgen (#192610)

Fixes #192408

You can tell an llvm build to use existing tools using options like
LLVM_NATIVE_TOOL_DIR.

When you do this, the tools are usually still built in case they need to
be part of a later install. For example if you provide a clang-tblgen,
we still build another clang-tblgen as part of the build.

Which means if you install that build, it is a complete install, and if
you are doing a cross build, you get a cross built copy of clang-tblgen
that you can use on the target machine.

In #159812, lldb-tblgen was added to the list of tools to install, so
that installed builds could be used as a base for subsequent builds.

However we were missing the EXPORT flag for lldb-tblgen. As far as I can
tell, this tells cmake that this tool will be part of the final install
regardless of whether something within the build requires it.

This missing option meant that cross builds, which do not have an
internal reason to build the cross lldb-tblgen binary, did not build it
did try to install it and of course failed because it did not exist.

Adding EXPORT forces lldb-tblgen to be built during the cross build and
now installing works as expected.

lldb/utils/TableGen/CMakeLists.txt

@@ -9,6 +9,7 @@ if (NOT DEFINED LLDB_TABLEGEN_EXE)
 
     add_tablegen(lldb-tblgen LLDB
       DESTINATION "${CMAKE_INSTALL_BINDIR}"
+      EXPORT LLDB
       LLDBOptionDefEmitter.cpp
       LLDBPropertyDefEmitter.cpp
       LLDBTableGen.cpp