[CMake][llvm] avoid includes with generator-expressions in add_custom_command

[radford]

Summary

Table generation's add_custom_command will break when any of the
INCLUDE_DIRECTORIES uses generator-expressions.

[JDevlieghere]

What's a scenario where you'd end up with a generator expression in tblgen_includes? I'm concerned that stripping generator expressions just hides the problem.

[radford]

When adding LLVM as a cmake sub-project in a larger project, the larger project might have (and did have in my case) unrelated-to-LLVM generator-expression-wrapped includes.

To be clear, add_custom_command *does* support generator-expressions, just not things like COMPILE_LANGUAGE which give an error like:

$<COMPILE_LANGUAGE:...> may only be used to specify include directories,
compile definitions, compile options, and to evaluate components of the
file(GENERATE) command.

So in my case this isn’t hiding a bigger problem, but it could conceivably in some cases.

llvm/cmake/modules/TableGen.cmake

function(tablegen project ofn)
   # a tablegen change, as cmake does not propagate file-level dependencies
   # of custom targets. See the following ticket for more information:
   # https://cmake.org/Bug/view.php?id=15858
   # The dependency on both, the target and the file, produces the same
   # dependency twice in the result file when
   # ("${${project}_TABLEGEN_TARGET}" STREQUAL "${${project}_TABLEGEN_EXE}")
   # but lets us having smaller and cleaner code here.
   get_directory_property(tblgen_includes INCLUDE_DIRECTORIES)
+  # No support for generator-expressions in add_custom_command, so strip them
+  list(TRANSFORM tblgen_includes GENEX_STRIP)
   list(TRANSFORM tblgen_includes PREPEND -I)
 
   add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
     COMMAND ${${project}_TABLEGEN_EXE} ${ARGN} -I ${CMAKE_CURRENT_SOURCE_DIR}
     ${tblgen_includes}
     ${LLVM_TABLEGEN_FLAGS}
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
     ${tblgen_change_flag}