[llvm] [Flang-RT][Offload] Always use LLVM-built GTest (PR #143682)

[Michał Górny via llvm-commits]

mgorny wrote:

This change broke building flang-rt against installed LLVM:

```
FAILED: flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/AccessTest.cpp.o 
/usr/lib/ccache/bin/x86_64-pc-linux-gnu-g++ -DNOT_EXE=\"/usr/lib/llvm/21/bin/not\" -D_DEBUG -D_GLIBCXX_ASSERTIONS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/var/tmp/portage/llvm-runtimes/flang-rt-21.0.0.9999/work/flang-rt/include -I/var/tmp/portage/llvm-runtimes/flang-rt-21.0.0.9999/work/flang-rt/../flang/include -isystem /usr/lib/llvm/21/include/llvm-gtest -isystem /usr/lib/llvm/21/include/llvm-gmock  -O2 -pipe -march=native -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -Wimplicit-fallthrough -Wno-nonnull -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -std=gnu++17 -UNDEBUG -Wno-suggest-override -U_GLIBCXX_ASSERTIONS -U_LIBCPP_ENABLE_ASSERTIONS -MD -MT flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/AccessTest.cpp.o -MF flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/AccessTest.cpp.o.d -o flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/AccessTest.cpp.o -c /var/tmp/portage/llvm-runtimes/flang-rt-21.0.0.9999/work/flang-rt/unittests/Runtime/AccessTest.cpp
In file included from /usr/lib/llvm/21/include/llvm-gtest/gtest/internal/custom/gtest-port.h:37,
                 from /usr/lib/llvm/21/include/llvm-gtest/gtest/internal/gtest-port.h:312,
                 from /usr/lib/llvm/21/include/llvm-gtest/gtest/gtest-message.h:57,
                 from /usr/lib/llvm/21/include/llvm-gtest/gtest/gtest-assertion-result.h:46,
                 from /usr/lib/llvm/21/include/llvm-gtest/gtest/gtest.h:64,
                 from /var/tmp/portage/llvm-runtimes/flang-rt-21.0.0.9999/work/flang-rt/unittests/Runtime/CrashHandlerFixture.h:16,
                 from /var/tmp/portage/llvm-runtimes/flang-rt-21.0.0.9999/work/flang-rt/unittests/Runtime/AccessTest.cpp:12:
/usr/lib/llvm/21/include/llvm-gtest/gtest/internal/custom/raw-ostream.h:43:10: fatal error: llvm/Support/raw_os_ostream.h: No such file or directory
   43 | #include "llvm/Support/raw_os_ostream.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
ninja: build stopped: subcommand failed.
```

I've filed #148362 with a proposed fix.

https://github.com/llvm/llvm-project/pull/143682