[BOLT] Drop BOLT_CLANG_EXE (#182)

As a follow-up to the issue unearthed by
llvm/llvm-project#89693, remove all uses of
BOLT_CLANG_EXE to prevent similar blockers in the future and reduce
builder
maintenance overhead.

Author: aaupov

buildbot/osuosl/master/config/builders.py

@@ -2702,11 +2702,10 @@
                         "-DLLVM_APPEND_VC_REV=OFF",
                         "-DCMAKE_C_COMPILER_LAUNCHER=ccache",
                         "-DCMAKE_CXX_COMPILER_LAUNCHER=ccache",
-                        "-DLLVM_ENABLE_PROJECTS=bolt;lld",
+                        "-DLLVM_ENABLE_PROJECTS=bolt;clang;lld",
                         "-DLLVM_TARGETS_TO_BUILD=X86;AArch64;RISCV",
                         "-DLLVM_LINK_LLVM_DYLIB=ON",
                         "-DLLVM_ENABLE_LLD=ON",
-                        "-DBOLT_CLANG_EXE=/usr/bin/clang",
                         ],
                     )},
 
@@ -2721,11 +2720,10 @@
                         "-DLLVM_APPEND_VC_REV=OFF",
                         "-DCMAKE_C_COMPILER_LAUNCHER=ccache",
                         "-DCMAKE_CXX_COMPILER_LAUNCHER=ccache",
-                        "-DLLVM_ENABLE_PROJECTS=bolt;lld",
+                        "-DLLVM_ENABLE_PROJECTS=bolt;clang;lld",
                         "-DLLVM_TARGETS_TO_BUILD=X86;AArch64;RISCV",
                         "-DBUILD_SHARED_LIBS=ON",
                         "-DLLVM_ENABLE_LLD=ON",
-                        "-DBOLT_CLANG_EXE=/usr/bin/clang",
                         ],
                     )},
 
@@ -2769,11 +2767,10 @@
                         "-DLLVM_APPEND_VC_REV=OFF",
                         "-DCMAKE_C_COMPILER_LAUNCHER=ccache",
                         "-DCMAKE_CXX_COMPILER_LAUNCHER=ccache",
-                        "-DLLVM_ENABLE_PROJECTS=bolt;lld",
+                        "-DLLVM_ENABLE_PROJECTS=bolt;clang;lld",
                         "-DLLVM_TARGETS_TO_BUILD=X86;AArch64;RISCV",
                         "-DBUILD_SHARED_LIBS=ON",
                         "-DLLVM_USE_LINKER=mold",
-                        "-DBOLT_CLANG_EXE=/usr/bin/clang",
                         ],
                     )},