[flang] Simplify LIBTYPE logic

[Meinersbur]

When the add_flang_library was first added, it was apparently copied over from add_clang_library, including its logic to determine the library type. It includes a workaround: If BUILD_SHARED_LIBS is enabled, it should build all libraries as shared, including those that are explicitly marked as STATIC¹, because add_clang_library always passes at least one of STATIC/SHARED to llvm_add_library, and llvm_add_library could not distinguish the two cases.

Then, the two implementations diverged. For its runtime libraries, Flang requires some libraries to always be static libraries, so if a library is explicitly marked as STATIC, BUILD_SHARED_LIBS is ignored².
 
I noticed the two implementations of the same functionality, modified only the add_clang_library, and copied over the result to add_flang_library³, without noticing that they are slightly different. As a result, Flang runtime libraries would be built as shared libraries with -DBUILD_SHARED_LIBS=ON, which may break some build configurations⁴.

This PR fixes the problem and at the same time simplifies the library type algorithm by just passing SHARED/STATIC verbatim to llvm_add_library. This is effectively what ² should have done instead adding more code to undo the workaround of ¹. Ideally, one would use

llvm_add_library(${name} ${ARG_STATIC} ${ARG_SHARED} [...])

but ARG_STATIC/ARG_SHARED as set by cmake_parse_arguments contain TRUE/FALSE instead of the keywords themselves. I could imagine a utility function akin to pythonize_bool that does this.

This simplification adds two more changes:

1. Object libraries are not explicitly requested anymore. llvm_add_library itself should determine whether an object library is necessary. As the comment notes, using an object library is not without problems and seem of no use here since it works fine without object library for XCODE/MSVC_IDE.

2. The property CLANG_STATIC_LIBS was removed. It was FLANG_STATIC_LIBS before to copy&paste error of Avoid object libraries in the VS IDE #93519 ³ which not used anywhere. In clang, CLANG_STATIC_LIBS is used for clang-shlib to include all component libraries in a single large library. There is no equivalent flang-shlib.

Footnotes

1. dbc2a12 ↩ ↩²

2. 3d2e05d ↩ ↩²

3. Avoid object libraries in the VS IDE #93519 ↩ ↩²

4. https://github.com/llvm/llvm-project/pull/93519#issuecomment-2192359002 ↩

[Meinersbur]

To elaborate on the idea of making forwarding of flags easier:

function(bool_argument_as_forwardable_flag prefix flag)
  if ("${${prefix}_${flag}}")
    set("${prefix}_${flag}" "${flag}" PARENT_SCOPE)
  else()
    set("${prefix}_${flag}" "" PARENT_SCOPE)
  endif()
endfunction()

bool_argument_as_forwardable_flag(ARG STATIC)
bool_argument_as_forwardable_flag(ARG SHARED)
llvm_add_library(${name} ${ARG_STATIC} ${ARG_SHARED} [...])

[dpalermo]

I have verified this fixes the issue with the llvm-flang build when the cmake flag is specified: -DFLANG_RUNTIME_F128_MATH_LIB=libquadmath