unittests: Build runtime unit tests with -DGTEST_NO_LLVM_SUPPORT

...to work around error building `unittests/runtime` on Linux.

`stdlib/include/llvm/Support` headers, a subset of
`llvm-project/llvm/include/llvm/Support`, are not compatible with the
latter.

Since we favor stdlib headers here, this makes sure Support headers will
not be transitively included through gtest, and helps to avoid mixed
includes, where a Support header existing only in LLVM is picked from
LLVM, whereas a Support header existing in both of the aforementioned
directories is picked from our stdlib.

Author: AnthonyLatsis

unittests/runtime/CMakeLists.txt

@@ -140,6 +140,18 @@ if(("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "${SWIFT_PRIMARY_VARIANT_SDK}") AND
   target_include_directories(SwiftRuntimeTests BEFORE PRIVATE
     ${SWIFT_SOURCE_DIR}/stdlib/public)
 
+  # `stdlib/include/llvm/Support` headers, a subset of
+  # `llvm-project/llvm/include/llvm/Support`, are not compatible with the
+  # latter.
+  #
+  # Since we favor stdlib headers here, this makes sure Support headers will not
+  # be transitively included through gtest, and helps to avoid mixed includes,
+  # where a Support header existing only in LLVM is picked from LLVM, whereas
+  # a Support header existing in both of the aforementioned directories is
+  # picked from our stdlib.
+  target_compile_definitions(SwiftRuntimeTests PRIVATE
+    GTEST_NO_LLVM_SUPPORT)
+
   # FIXME: cross-compile for all variants.
   target_link_libraries(SwiftRuntimeTests
     PRIVATE