[BOLT][CMAKE] Use BOLT_CLANG_EXE and BOLT_LLD_EXE as is

[Amir]

Summary

Add an ability to provide paths that don't match tool name exactly:
e.g. clang-13.
Remove use_lld call that sets up unused extra tools.

Test plan:

cmake -G Ninja ../llvm-project/llvm -DLLVM_TARGETS_TO_BUILD="X86;AArch64" -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="bolt" -DBOLT_CLANG_EXE=/usr/bin/clang-13 -DBOLT_LLD_EXE=/usr/bin/lld-13 
...
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using clang: /usr/bin/clang-13
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using lld: /usr/bin/lld-13

cmake -G Ninja ../llvm-project/llvm -DLLVM_TARGETS_TO_BUILD="X86;AArch64" -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="bolt;lld" -DBOLT_CLANG_EXE=/usr/bin/clang-13
...
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using clang: /usr/bin/clang-13
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using lld: /data/llvm-build2/bin/lld

cmake -G Ninja ../llvm-project/llvm -DLLVM_TARGETS_TO_BUILD="X86;AArch64" -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="bolt;clang;lld" 
...
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using clang: /data/llvm-build3/bin/clang
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using lld: /data/llvm-build3/bin/lld

[Amir]

Use CLANG and LLD environment variables, removing the dependence on
https://reviews.llvm.org/D117445

bolt/test/lit.cfg.py

else:
     lit_config.warning('Setting a timeout per test not supported. ' + errormsg
                        + ' Some tests will be skipped.')
 
 if config.bolt_enable_runtime:
     config.available_features.add("bolt-runtime")
 
 llvm_config.use_default_substitutions()
 
-llvm_config.use_clang(additional_tool_dirs=[os.path.dirname(config.bolt_clang)])
-llvm_config.use_lld(additional_tool_dirs=[os.path.dirname(config.bolt_lld)])
+llvm_config.config.environment['CLANG'] = config.bolt_clang
+llvm_config.config.environment['LLD'] = config.bolt_lld
+llvm_config.use_clang()
+llvm_config.use_llvm_tool('lld', required=True, search_env='LLD')
 
 config.substitutions.append(('%cflags', '-no-pie'))
 config.substitutions.append(('%cxxflags', '-no-pie'))
 
 link_fdata_cmd = os.path.join(config.test_source_root, 'link_fdata.py')
 
 tool_dirs = [config.llvm_tools_dir,
              config.test_source_root]