[cmake] Do not issue a fatal error on version mismatch for Debug builds.

Instead we print a warning presuming the developer knows what to do. The use
case is upgrading clad to newer llvm versions.

CMakeLists.txt

@@ -103,7 +103,11 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   endif()
 
   if (LLVM_PACKAGE_VERSION VERSION_LESS LLVM_MIN_SUPPORTED OR LLVM_PACKAGE_VERSION VERSION_GREATER_EQUAL LLVM_VERSION_UPPER_BOUND)
-    message(FATAL_ERROR "Found unsupported version: LLVM ${LLVM_PACKAGE_VERSION};\nPlease set LLVM_DIR pointing to the llvm version ${LLVM_MIN_SUPPORTED} to ${LLVM_MAX_SUPPORTED} build or installation folder")
+    set(diag_kind FATAL_ERROR)
+    if (CMAKE_BUILD_TYPE STREQUAL "Debug")
+      set(diag_kind WARNING)
+    endif()
+    message(${diag_kind} "Found unsupported version: LLVM ${LLVM_PACKAGE_VERSION};\nPlease set LLVM_DIR pointing to the llvm version ${LLVM_MIN_SUPPORTED} to ${LLVM_MAX_SUPPORTED} build or installation folder")
   endif()
 
   message(STATUS "Found supported version: LLVM ${LLVM_PACKAGE_VERSION}")
@@ -136,7 +140,11 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   endif()
 
   if (LLVM_PACKAGE_VERSION VERSION_LESS CLANG_MIN_SUPPORTED OR LLVM_PACKAGE_VERSION VERSION_GREATER_EQUAL CLANG_VERSION_UPPER_BOUND)
-    message(FATAL_ERROR "Found unsupported version: Clang ${LLVM_PACKAGE_VERSION};\nPlease set Clang_DIR pointing to the clang version ${CLANG_MIN_SUPPORTED} to ${CLANG_MAX_SUPPORTED} build or installation folder")
+    set(diag_kind FATAL_ERROR)
+    if (CMAKE_BUILD_TYPE STREQUAL "Debug")
+      set(diag_kind WARNING)
+    endif()
+    message(${diag_kind} "Found unsupported version: Clang ${LLVM_PACKAGE_VERSION};\nPlease set Clang_DIR pointing to the clang version ${CLANG_MIN_SUPPORTED} to ${CLANG_MAX_SUPPORTED} build or installation folder")
   endif()
 
   message(STATUS "Found supported version: Clang ${LLVM_PACKAGE_VERSION}")