[llvm-branch-commits] [runtimes] Allow building against an installed LLVM tree (PR #86209)

[Alexander Richardson via llvm-branch-commits]

https://github.com/arichardson created https://github.com/llvm/llvm-project/pull/86209

I am currently trying to test the LLVM runtimes (including compiler-rt)
against an installed LLVM tree rather than a build tree (since that is
no longer available). Currently, the runtimes build of compiler-rt assumes
that LLVM_BINARY_DIR is writable since it uses configure_file() to write
there during the CMake configure stage. Instead, generate this file inside
CMAKE_CURRENT_BINARY_DIR, which will match LLVM_BINARY_DIR when invoked
from llvm/runtimes/CMakeLists.txt.

I also needed to make a minor change to the hwasan tests: hwasan_symbolize
was previously found in the LLVM_BINARY_DIR, but since it is generated as
part of the compiler-rt build it is now inside the CMake build directory
instead. I fixed this by passing the output directory to lit as
config.compiler_rt_bindir and using llvm_config.add_tool_substitutions().

For testing that we no longer write to the LLVM install directory as
part of testing or configuration, I created a read-only bind mount and
configured the runtimes builds as follows:
```
$ sudo mount --bind --read-only ~/llvm-install /tmp/upstream-llvm-readonly
$ cmake -DCMAKE_BUILD_TYPE=Debug \
  -DCMAKE_C_COMPILER=/tmp/upstream-llvm-readonly/bin/clang \
  -DCMAKE_CXX_COMPILER=/tmp/upstream-llvm-readonly/bin/clang++ \
  -DLLVM_INCLUDE_TESTS=TRUE -DLLVM_ENABLE_ASSERTIONS=TRUE \
  -DCOMPILER_RT_INCLUDE_TESTS=TRUE -DCOMPILER_RT_DEBUG=OFF \
  -DLLVM_ENABLE_RUNTIMES=compiler-rt \
  -DLLVM_BINARY_DIR=/tmp/upstream-llvm-readonly \
  -G Ninja -S ~/upstream-llvm-project/runtimes \
  -B ~/upstream-llvm-project/runtimes/cmake-build-debug-llvm-git
```