[runtimes][GTest] LLVM-independent unittests (#164794)

The LLVM-customized GTest has a dependency on LLVM to support
`llvm::raw_ostream` and hence has to link to LLVMSupport. The runtimes
use the LLVMSupport from the bootstrapping LLVM build. The problem is
that the boostrapping compiler and the runtimes target can diverge in
their ABI, even in the runtimes default build. For instance, Clang is
built using gcc which uses libstdc++, but the runtimes is built by Clang
which can be configured to use libcxx by default. Altough it does not
use gcc, this issue has caused
[flang-aarch64-libcxx](https://lab.llvm.org/buildbot/#/builders/89)) to
break, and is still (again?) broken.

This patch makes the runtimes' GTest independent from LLVMSupport so we
do not link any runtimes component with LLVM components.

Runtime projects that use GTest unittests:
 * flang-rt
 * libc
* compiler-rt: Adds `gtest-all.cpp` with
[GTEST_NO_LLVM_SUPPORT=1](https://github.com/llvm/llvm-project/blob/f801b6f67ea896d6e4d2de38bce9a79689ceb254/compiler-rt/CMakeLists.txt#L723)
to each unittest without using `llvm_gtest`. Not touched by this PR.
* openmp: Handled by #159416. Not touched for now by this PR to avoid
conflict.

The current state of this PR tries to reuse
https://github.com/llvm/llvm-project/blob/main/third-party/unittest/CMakeLists.txt
as much as possible, altough personally I would prefer to make it use
"modern CMake" style. third-party/unittest/CMakeLists.txt will detect
whether it is used in runtimes build and adjaust accordingly. It creates
a different target for LLVM (`llvm_gtest`, NFCI) and another one for the
runtimes (`runtimes_gtest`). It is not possible to reuse `llvm_gtest`
for both since `llvm_gtest` is imported using `find_package(LLVM)` if
configured using LLVM_INSTALL_GTEST. An alias `default_gtest` is used to
select between the two. `default_gtest` could also be used for openmp
which also supports standalone and
[LLVM_ENABLE_PROJECTS](https://github.com/llvm/llvm-project/pull/152189)
build mode.

1 files changed, 14 insertions, 0 deletions

diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
index cc756da426e6..b17bd8f66536 100644
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -256,6 +256,20 @@ endif()
 # This can be used to detect whether we're in the runtimes build.
 set(LLVM_RUNTIMES_BUILD ON)
 
+# Make GTest available to all runtimes
+# The runtime must call make_gtest_target() for it to become available. This is
+# to avoid build failures when gtest is not actually needed. In particular,
+# mingw-incomplete-sysroot is missing C++ header files that GTest needs to
+# compile.
+function(build_gtest)
+  if(TARGET default_gtest)
+    # Already available
+    return()
+  endif()
+
+  add_subdirectory("${LLVM_THIRD_PARTY_DIR}/unittest" "${CMAKE_BINARY_DIR}/third-party/runtimes_gtest")
+endfunction()
+
 foreach(entry ${runtimes})
   get_filename_component(projName ${entry} NAME)