[Fuchsia] Add FUCHSIA_USE_MULTIPLE_DISTRIBUTIONS.

[mysterymath]

Summary

This flag causes the toolchain distribution to be built using LLVM
CMake's multiple distribution feature. The distribution* family of CMake
targets would be replaced with the toolchain-distribution* family.

This shouldn't otherwise affect the semantics of the build, but it sets
up the ability to split out the LLDB build from the main distribution
used by Fuchsia.

[phosek]

clang/cmake/caches/Fuchsia-stage2.cmake

3

Could we use multiple distributions unconditionally?

[mysterymath]

clang/cmake/caches/Fuchsia-stage2.cmake

3

Yep, this is just the feature flag mechanism to prevent breaking the build and provide a safe revert. As soon as this and its partner change are stable, I'll yank the flag.

[phosek]

LGTM

clang/cmake/caches/Fuchsia-stage2.cmake

 # This file sets up a CMakeCache for the second stage of a Fuchsia toolchain build.
 
+option(FUCHSIA_USE_MULTIPLE_DISTRIBUTIONS "Use multiple distributions")
+option(FUCHSIA_ENABLE_LLDB "Enable LLDB")
+
 set(LLVM_TARGETS_TO_BUILD X86;ARM;AArch64;RISCV CACHE STRING "")
 
 set(PACKAGE_VENDOR Fuchsia CACHE STRING "")
 
 set(_FUCHSIA_ENABLE_PROJECTS "bolt;clang;clang-tools-extra;lld;llvm;polly")
 set(LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "")
 
 set(LLVM_ENABLE_BACKTRACES OFF CACHE BOOL "")
set(LLVM_TOOLCHAIN_TOOLS
   llvm-strip
   llvm-symbolizer
   llvm-undname
   llvm-xray
   sancov
   scan-build-py
   CACHE STRING "")
 
-set(_FUCHSIA_DISTRIBUTION_COMPONENTS
+set(LLVM_Toolchain_DISTRIBUTION_COMPONENTS
   clang
   lld
   clang-apply-replacements
   clang-doc
   clang-format
   clang-resource-headers
   clang-include-fixer
   clang-refactor
   clang-scan-deps
   clang-tidy
   clangd
   find-all-symbols
   builtins
   runtimes
-  ${LLVM_TOOLCHAIN_TOOLS})
+  ${LLVM_TOOLCHAIN_TOOLS}
+  CACHE STRING "")
+
+if(FUCHSIA_USE_MULTIPLE_DISTRIBUTIONS)
+  set(_FUCHSIA_DISTRIBUTIONS Toolchain)
+else()
+  set(LLVM_DISTRIBUTION_COMPONENTS ${LLVM_Toolchain_DISTRIBUTION_COMPONENTS} CACHE STRING "")
+endif()
 
-set(FUCHSIA_ENABLE_LLDB OFF CACHE BOOL "Enable LLDB")
 if(FUCHSIA_ENABLE_LLDB)
+  if(NOT FUCHSIA_USE_MULTIPLE_DISTRIBUTIONS)
+    message(FATAL_ERROR "FUCHSIA_ENABLE_LLDB requires FUCHSIA_USE_MULTIPLE_DISTRIBUTIONS.")
+  endif()
   list(APPEND _FUCHSIA_ENABLE_PROJECTS lldb)
-  list(APPEND _FUCHSIA_DISTRIBUTION_COMPONENTS lldb liblldb lldb-server lldb-argdumper)
+  list(APPEND _FUCHSIA_DISTRIBUTIONS Debugger)
+  set(LLVM_Debugger_DISTRIBUTION_COMPONENTS
+    lldb
+    liblldb
+    lldb-server
+    lldb-argdumper
+    CACHE STRING "")
+endif()
+
+if(FUCHSIA_USE_MULTIPLE_DISTRIBUTIONS)
+  set(LLVM_DISTRIBUTIONS ${_FUCHSIA_DISTRIBUTIONS} CACHE STRING "")
 endif()
 
 set(LLVM_ENABLE_PROJECTS ${_FUCHSIA_ENABLE_PROJECTS} CACHE STRING "")
-set(LLVM_DISTRIBUTION_COMPONENTS ${_FUCHSIA_DISTRIBUTION_COMPONENTS} CACHE STRING "")

clang/cmake/caches/Fuchsia.cmake

 # This file sets up a CMakeCache for a Fuchsia toolchain build.
 
+option(FUCHSIA_USE_MULTIPLE_DISTRIBUTIONS "Use multiple distributions")
+option(FUCHSIA_ENABLE_LLDB "Enable LLDB")
+
 set(LLVM_TARGETS_TO_BUILD X86;ARM;AArch64;RISCV CACHE STRING "")
 
 set(PACKAGE_VENDOR Fuchsia CACHE STRING "")
 
 set(_FUCHSIA_ENABLE_PROJECTS "bolt;clang;clang-tools-extra;lld;llvm;polly")
 
 set(LLVM_ENABLE_DIA_SDK OFF CACHE BOOL "")
 set(LLVM_ENABLE_LIBEDIT OFF CACHE BOOL "")
set(_FUCHSIA_BOOTSTRAP_PASSTHROUGH
   LLVM_ENABLE_ZSTD
   zstd_DIR
   LLVM_ENABLE_LIBXML2
   LibXml2_ROOT
   LLVM_ENABLE_CURL
   CURL_ROOT
   OpenSSL_ROOT
   FUCHSIA_ENABLE_LLDB
+  FUCHSIA_USE_MULTIPLE_DISTRIBUTIONS
   LLDB_ENABLE_CURSES
   LLDB_ENABLE_LIBEDIT
   CMAKE_FIND_PACKAGE_PREFER_CONFIG
   CMAKE_SYSROOT
   CMAKE_MODULE_LINKER_FLAGS
   CMAKE_SHARED_LINKER_FLAGS
   CMAKE_EXE_LINKER_FLAGS
   LLVM_WINSYSROOT
set(_FUCHSIA_BOOTSTRAP_TARGETS
   check-llvm
   check-polly
   llvm-config
   clang-test-depends
   lld-test-depends
   llvm-test-depends
   test-suite
   test-depends
+  clang)
+
+if(FUCHSIA_USE_MULTIPLE_DISTRIBUTIONS)
+  list(APPEND _FUCHSIA_BOOTSTRAP_TARGETS
+    toolchain-distribution
+    install-toolchain-distribution
+    install-toolchain-distribution-stripped
+    install-toolchain-distribution-toolchain)
+else()
+  list(APPEND _FUCHSIA_BOOTSTRAP_TARGETS
     distribution
     install-distribution
     install-distribution-stripped
-  install-distribution-toolchain
-  clang)
+    install-distribution-toolchain)
+endif()
 
-set(FUCHSIA_ENABLE_LLDB OFF CACHE BOOL "Enable LLDB")
 if(FUCHSIA_ENABLE_LLDB)
   list(APPEND _FUCHSIA_ENABLE_PROJECTS lldb)
-  list(APPEND _FUCHSIA_BOOTSTRAP_TARGETS check-lldb lldb-test-depends)
+  list(APPEND _FUCHSIA_BOOTSTRAP_TARGETS
+    check-lldb
+    lldb-test-depends
+    debugger-distribution
+    install-debugger-distribution
+    install-debugger-distribution-stripped
+    install-debugger-distribution-toolchain)
 endif()
+
 set(LLVM_ENABLE_PROJECTS ${_FUCHSIA_ENABLE_PROJECTS} CACHE STRING "")
 set(CLANG_BOOTSTRAP_TARGETS ${_FUCHSIA_BOOTSTRAP_TARGETS} CACHE STRING "")
 
 get_cmake_property(variableNames VARIABLES)
 foreach(variableName ${variableNames})
   if(variableName MATCHES "^STAGE2_")
     string(REPLACE "STAGE2_" "" new_name ${variableName})
     list(APPEND EXTRA_ARGS "-D${new_name}=${${variableName}}")