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.
Reviewed By: lntue
Differential Revision: https://reviews.llvm.org/D139348
1dfb0cd703