[libc] Fix wrappergen_test.cpp on runtimes build

[jhuber6]

Summary

The tools directory depends on llvm_gtest which is not immediately
availible with a runtimes build. This patch builds the llvm_gtest
target if it isn't present. Additionally, we use the CMake binary
directory to find the tool binary, which is different when using a
runtimes build. Using the LLVM binary directory should match both build
conditions.

libc/test/utils/tools/CMakeLists.txt

COMMAND $<TARGET_FILE:${target_name}>
             ${LIBC_TOOL_UNITTEST_ARGS}
   )
   add_dependencies(libc-tool-unittests ${target_name})
 
   target_compile_options(${target_name} PUBLIC -fno-rtti)
   target_link_libraries(${target_name} PRIVATE LLVMSupport)
 endfunction()
 
+# Build the gtest library needed for unittests if we do not have it already.
+if (EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest AND NOT TARGET llvm_gtest)
+  include_directories(${LLVM_LIBC_INCLUDE_DIRS})
+  add_subdirectory(${LLVM_THIRD_PARTY_DIR}/unittest third-party/unittest)
+endif()
+
 add_subdirectory(WrapperGen)

libc/test/utils/tools/WrapperGen/CMakeLists.txt

 add_libc_tool_unittest(
   wrappergen_test
   SRCS 
     wrappergen_test.cpp   
   ARGS
     --path=${LIBC_SOURCE_DIR}
-    --tool=${CMAKE_BINARY_DIR}/bin/libc-wrappergen
+    --tool=${LLVM_BINARY_DIR}/bin/libc-wrappergen
     --api=${LIBC_SOURCE_DIR}/test/utils/tools/WrapperGen/testapi.td
 )
 
 add_dependencies(wrappergen_test libc-wrappergen)