[BOLT][CMAKE] Accept BOLT_CLANG_EXE and BOLT_LLD_EXE

Add CMake options to supply clang and lld binaries for use in check-bolt
instead of requiring the build of clang and lld projects.

Suggested by Mehdi Amini in https://lists.llvm.org/pipermail/llvm-dev/2021-December/154426.html

Test Plan:
```
cmake -G Ninja ~/local/llvm-project/llvm \
-DLLVM_TARGETS_TO_BUILD="X86"  \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_ENABLE_ASSERTIONS=ON \
-DLLVM_ENABLE_PROJECTS="bolt"  \
-DBOLT_CLANG_EXE=~/local/bin/clang \
-DBOLT_LLD_EXE=~/local/bin/lld

ninja check-bolt
...
llvm-lit: /home/aaupov/local/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using clang: /home/aaupov/local/bin/clang
llvm-lit: /home/aaupov/local/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using ld.lld: /home/aaupov/local/bin/ld.lld
llvm-lit: /home/aaupov/local/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using lld-link: /home/aaupov/local/bin/lld-link
llvm-lit: /home/aaupov/local/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using ld64.lld: /home/aaupov/local/bin/ld64.lld
llvm-lit: /home/aaupov/local/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using wasm-ld: /home/aaupov/local/bin/wasm-ld
...
```

Tested all configurations:
- LLVM_ENABLE_PROJECTS="bolt;clang;lld" + no BOLT_*_EXE
- LLVM_ENABLE_PROJECTS="bolt;clang" + BOLT_LLD_EXE
- LLVM_ENABLE_PROJECTS="bolt;lld" + BOLT_CLANG_EXE
- LLVM_ENABLE_PROJECTS="bolt" + BOLT_CLANG_EXE + BOLT_LLD_EXE
- LLVM_ENABLE_PROJECTS="bolt;clang;lld" + BOLT_CLANG_EXE + BOLT_LLD_EXE

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D117061