llvmPackages_12: reorganize files

Restructure pkgs/development/compilers/llvm/12 in a similar spirit
as #117433 to mirror the other llvm directories.

pkgs/development/compilers/llvm/12/default.nix

@@ -32,7 +32,7 @@ let
     '';
   in {
 
-    llvm = callPackage ./llvm.nix { };
+    llvm = callPackage ./llvm { };
 
     clang-unwrapped = callPackage ./clang {
       inherit (tools) lld;
@@ -80,11 +80,11 @@ let
       extraBuildCommands = mkExtraBuildCommands cc;
     };
 
-    lld = callPackage ./lld.nix {
+    lld = callPackage ./lld {
       libunwind = libraries.libunwind;
     };
 
-    lldb = callPackage ./lldb.nix {
+    lldb = callPackage ./lldb {
       inherit (darwin) libobjc bootstrap_cmds;
       inherit (darwin.apple_sdk.libs) xpc;
       inherit (darwin.apple_sdk.frameworks) Foundation Carbon Cocoa;
@@ -172,7 +172,7 @@ let
     callPackage = newScope (libraries // buildLlvmTools // { inherit stdenv cmake libxml2 python3 isl release_version version fetch; });
   in {
 
-    compiler-rt = callPackage ./compiler-rt.nix ({} //
+    compiler-rt = callPackage ./compiler-rt ({} //
       (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
         stdenv = overrideCC stdenv buildLlvmTools.lldClangNoCompilerRt;
       }));
@@ -186,7 +186,7 @@ let
         stdenv = overrideCC stdenv buildLlvmTools.lldClangNoLibcxx;
       }));
 
-    libcxxabi = callPackage ./libc++abi.nix ({} //
+    libcxxabi = callPackage ./libc++abi ({} //
       (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
         stdenv = overrideCC stdenv buildLlvmTools.lldClangNoLibcxx;
         libunwind = libraries.libunwind;
@@ -194,7 +194,7 @@ let
 
     openmp = callPackage ./openmp.nix {};
 
-    libunwind = callPackage ./libunwind.nix ({} //
+    libunwind = callPackage ./libunwind ({} //
       (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
         stdenv = overrideCC stdenv buildLlvmTools.lldClangNoLibcxx;
       }));