[gn] Use label_name rather than target_output_name for objects

[phosek]

Summary

Multiple targets in the same output directory can use the same
target_output_name. The typical example of that is having a shared
and a static library of the same, e.g. libc++.so and libc++.a.
When that's the case, the object files produced for each target
are going to conflict. Using the label_name avoids this conflict
since labels are guaranteed to be unique within a single BUILD.gn
file which corresponds to a single output directory.

[phosek]

Alternative I also considered would be to use {{source_out_dir}}/{{label_name}}.dir/{{source_name_part}}.o which is what CMake does, the advantage is that you end up with prettier object file names (i.e. no label_name or target_output_name prefixes) which might matter for distribution archives like libc++.a.

[phosek]

Ping, this is a pre-requisite for D60370 and D60372.

[phosek]

Ping again?

llvm/trunk/utils/gn/build/toolchain/BUILD.gn

toolchain(target_name) {
     forward_variables_from(invoker, "*")
 
     tool("cc") {
       depfile = "{{output}}.d"
       command = "$cc -MMD -MF $depfile -o {{output}} -c {{source}} {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}}"
       depsformat = "gcc"
       description = "CC {{output}}"
       outputs = [
-        "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
+        "{{source_out_dir}}/{{label_name}}.{{source_name_part}}.o"
       ]
     }
 
     tool("cxx") {
       depfile = "{{output}}.d"
       command = "$cxx -MMD -MF $depfile -o {{output}} -c {{source}} {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}}"
       depsformat = "gcc"
       description = "CXX {{output}}"
       outputs = [
-        "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
+        "{{source_out_dir}}/{{label_name}}.{{source_name_part}}.o"
       ]
     }
 
     tool("asm") {
       depfile = "{{output}}.d"
       command = "$cc -MMD -MF $depfile -o {{output}} -c {{source}} {{defines}} {{include_dirs}} {{asmflags}}"
       depsformat = "gcc"
       description = "ASM {{output}}"
       outputs = [
-        "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
+        "{{source_out_dir}}/{{label_name}}.{{source_name_part}}.o"
       ]
     }
 
     tool("alink") {
       if (current_os == "mac") {
         command = "libtool -static -no_warning_for_no_symbols {{arflags}} -o {{output}} {{inputs}}"
       } else {
         # Remove the output file first so that ar doesn't try to modify the
if (use_goma) {
     cl = "$goma_dir/gomacc $cl"
   }
 
   tool("cc") {
     command = "$cl /nologo /showIncludes /Fo{{output}} /c {{source}} {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}}"
     depsformat = "msvc"
     description = "CC {{output}}"
     outputs = [
-      "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.obj",
+      "{{source_out_dir}}/{{label_name}}.{{source_name_part}}.obj",
     ]
   }
 
   tool("cxx") {
     command = "$cl /nologo /showIncludes /Fo{{output}} /c {{source}} {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}}"
     depsformat = "msvc"
     description = "CXX {{output}}"
     outputs = [
-      "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.obj",
+      "{{source_out_dir}}/{{label_name}}.{{source_name_part}}.obj",
     ]
   }
 
   tool("alink") {
     command = "$link /lib /nologo {{arflags}} /out:{{output}} {{inputs}}"
     description = "LIB {{output}}"
     outputs = [
       "{{output_dir}}/{{target_output_name}}.lib",