[FLANG] Fix MSVC + clang-cl build

[omjavaid]

Summary

Flang build on windows with MSVC environment and clang-cl compiler
requires clang_rt.builtin.${target} library. This patch allows us to
locate and include this link library. This is mostly needed for flang
runtime and associated unittests. Clang implicitly uses this library
while calling lld-link via clang driver however a standalone call to
lld-link doesnt link against clang_rt library on its own.

[clementval]

How come this has landed without review?

[omjavaid]

Landed accidentally. Reverted the change.

[omjavaid]

ping!

[kiranchandramohan]

Would you know why we cannot see the results of the pre-commit CI?

[omjavaid]

In D137112#3914651, @kiranchandramohan wrote:

Would you know why we cannot see the results of the pre-commit CI?

Not sure why pre-commit CI is not triggered for this? Should I send a new change and discard this one. BTW, I used arc diff to create and submit this change.

[kiranchandramohan]

In D137112#3927117, @omjavaid wrote:

In D137112#3914651, @kiranchandramohan wrote:

Would you know why we cannot see the results of the pre-commit CI?

Not sure why pre-commit CI is not triggered for this? Should I send a new change and discard this one. BTW, I used arc diff to create and submit this change.

I also don't know why the CI was not triggered. If it is alrite, please send a new change.

flang/CMakeLists.txt

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   set(FLANG_STANDALONE_BUILD ON)
 else()
   set(FLANG_STANDALONE_BUILD OFF)
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
+# MSVC + clang-cl build requires clang_rt.builtin.${target} library
+if (MSVC)
+  include(HandleCompilerRT)
+  find_compiler_rt_library(builtins CLANG_RT_BUILTINS_LIBRARY)
+  get_filename_component(LIBDIR "${CLANG_RT_BUILTINS_LIBRARY}" DIRECTORY)
+  if (IS_DIRECTORY "${LIBDIR}")
+    link_libraries(${CLANG_RT_BUILTINS_LIBRARY})
+  endif()
+endif()
+
 if (FLANG_STANDALONE_BUILD)
   set(FLANG_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
   if (NOT MSVC_IDE)
     set(LLVM_ENABLE_ASSERTIONS ${ENABLE_ASSERTIONS}
       CACHE BOOL "Enable assertions")
     # Assertions follow llvm's configuration.
     mark_as_advanced(LLVM_ENABLE_ASSERTIONS)
   endif()