[llvm][Cmake] Expand "all" in LLVM_ENABLE_PROJECTS before other checks

llvm/CMakeLists.txt

 # one for llvm+clang+... using the same sources.
 set(LLVM_ALL_PROJECTS "bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;libcxx;libcxxabi;libunwind;lld;lldb;mlir;openmp;polly;pstl")
 # The flang project is not yet part of "all" projects (see C++ requirements)
 set(LLVM_EXTRA_PROJECTS "flang")
 # List of all known projects in the mono repo
 set(LLVM_KNOWN_PROJECTS "${LLVM_ALL_PROJECTS};${LLVM_EXTRA_PROJECTS}")
 set(LLVM_ENABLE_PROJECTS "" CACHE STRING
     "Semicolon-separated list of projects to build (${LLVM_KNOWN_PROJECTS}), or \"all\".")
+# Make sure expansion happens first to not handle "all" in rest of the checks.
+if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
+  set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})
+endif()
 foreach(proj ${LLVM_ENABLE_PROJECTS})
-  if (NOT proj STREQUAL "all" AND NOT proj STREQUAL "llvm" AND NOT "${proj}" IN_LIST LLVM_KNOWN_PROJECTS)
+  if (NOT proj STREQUAL "llvm" AND NOT "${proj}" IN_LIST LLVM_KNOWN_PROJECTS)
      MESSAGE(FATAL_ERROR "${proj} isn't a known project: ${LLVM_KNOWN_PROJECTS}")
   endif()
 endforeach()
 foreach(proj "libcxx" "libcxxabi" "libunwind")
   if (${proj} IN_LIST LLVM_ENABLE_PROJECTS)
     # TODO(Ericson2314): Making non-fatal for now because of out-of-tree docs
     # CI jobs use this.
     message(SEND_ERROR
       "Using LLVM_ENABLE_PROJECTS=${proj} is incorrect. Please use -DLLVM_ENABLE_RUNTIMES=${proj} or "
       "see the instructions at https://libcxx.llvm.org/BuildingLibcxx.html for building the runtimes.")
   endif()
 endforeach()
 
-if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
-  set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})
-endif()
-
 if ("flang" IN_LIST LLVM_ENABLE_PROJECTS)
   if (NOT "mlir" IN_LIST LLVM_ENABLE_PROJECTS)
     message(STATUS "Enabling MLIR as a dependency to flang")
     list(APPEND LLVM_ENABLE_PROJECTS "mlir")
   endif()
 
   if (NOT "clang" IN_LIST LLVM_ENABLE_PROJECTS)
     message(FATAL_ERROR "Clang is not enabled, but is required for the Flang driver")