[Fuchsia] Add FUCHSIA_ENABLE_LLDB option.

[mysterymath]

Summary

This CMake option builds/installs LLDB as part of the Fuchsia toolchain.
Once this is better supported, the effects of this will be inlined into
the toolchain cache file.

[mysterymath]

_FUCHSIA_DISTRIBUTION_COMPONENTS should not be a CACHE variable.

[phosek]

LGTM

clang/cmake/caches/Fuchsia-stage2.cmake

 # This file sets up a CMakeCache for the second stage of a Fuchsia toolchain build.
 
 set(LLVM_TARGETS_TO_BUILD X86;ARM;AArch64;RISCV CACHE STRING "")
 
 set(PACKAGE_VENDOR Fuchsia CACHE STRING "")
 
-set(LLVM_ENABLE_PROJECTS "bolt;clang;clang-tools-extra;lld;llvm;polly" 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_ENABLE_DIA_SDK OFF CACHE BOOL "")
 set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
 set(LLVM_ENABLE_LIBEDIT OFF CACHE BOOL "")
 set(LLVM_ENABLE_LLD ON CACHE BOOL "")
 set(LLVM_ENABLE_LTO ON CACHE BOOL "")
set(LLVM_TOOLCHAIN_TOOLS
   llvm-strip
   llvm-symbolizer
   llvm-undname
   llvm-xray
   sancov
   scan-build-py
   CACHE STRING "")
 
-set(LLVM_DISTRIBUTION_COMPONENTS
+set(_FUCHSIA_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}
   CACHE STRING "")
+
+set(FUCHSIA_ENABLE_LLDB OFF CACHE BOOL "Enable LLDB")
+if(FUCHSIA_ENABLE_LLDB)
+  list(APPEND _FUCHSIA_ENABLE_PROJECTS "lldb")
+  list(APPEND _FUCHSIA_DISTRIBUTION_COMPONENTS "lldb")
+endif()
+
+set(LLVM_ENABLE_PROJECTS ${_FUCHSIA_ENABLE_PROJECTS} CACHE STRING "")
+set(LLVM_DISTRIBUTION_COMPONENTS ${_FUCHSIA_DISTRIBUTION_COMPONENTS} CACHE STRING "")