[Support][Driver][LLDB] Guard WIN32_LEAN_AND_MEAN definitions with #ifndef (#213045)

Several Windows headers/TUs unconditionally `#define
WIN32_LEAN_AND_MEAN`. When a build predefines the macro on the compiler
command line (e.g. a toolchain that passes -DWIN32_LEAN_AND_MEAN, which
clang treats as `#define WIN32_LEAN_AND_MEAN 1`), the differing token
lists trigger -Wmacro-redefined, which becomes a hard error under
-Werror.

Guard the definitions with #ifndef, matching the adjacent NOMINMAX
handling and the existing pattern in
llvm/lib/Support/rpmalloc/rpmalloc.c. The macro is only a presence flag,
so keeping an externally-provided definition is correct.

Sites guarded:
  llvm/include/llvm/Support/Windows/WindowsSupport.h
  llvm/lib/WindowsDriver/MSVCPaths.cpp
  clang/lib/Driver/ToolChains/MSVC.cpp
  lldb/include/lldb/Host/windows/windows.h

Meta ran into this internally building lldb on window after a sync with
upstream from July commit, so adding guards would allow us to drop a
workaround of ignoring the duplicate defines.

Error:
llvm\include\llvm/Support/Windows/WindowsSupport.h(29,9): error:
'WIN32_LEAN_AND_MEAN' macro redefined [-Werror,-Wmacro-redefined]
   29 | #define WIN32_LEAN_AND_MEAN
      |         ^
<command line>(10,9): note: previous definition is here
   10 | #define WIN32_LEAN_AND_MEAN 1
      |         ^
1 error generated.
GitOrigin-RevId: 03f86c6d4d55809f9531b5e37eb8249e569746aa