[gn build] Use build-machine-independent paths in coverage information

[thakis]

Summary

This is possible after D106314 / 8773822c578a.

Makes the required prepare-code-coverage-artifact.py invocation a bit longer,
but that seems like a good tradeoff.

[hans]

lgtm

llvm/utils/gn/build/BUILD.gn

 import("//llvm/utils/gn/build/buildflags.gni")
 import("//llvm/utils/gn/build/mac_sdk.gni")
 import("//llvm/utils/gn/build/sysroot.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 import("//llvm/utils/gn/build/toolchain/target_flags.gni")
 
 declare_args() {
   # Whether to build everything with coverage information.
   # After building with this, run tests and then run
-  #    llvm/utils/prepare-code-coverage-artifact.py  \
+  #    llvm/utils/prepare-code-coverage-artifact.py \
+  #        --compilation-dir=out/gn \
   #        .../llvm-profdata .../llvm-cov out/gn/profiles/ report/ \
   #        out/gn/bin/llvm-undname ...
   # to generate a HTML report for the binaries passed in the last line.
   llvm_build_instrumented_coverage = false
 
   # If set, puts relative paths in debug info.
   # Makes the build output independent of the build directory, but makes
   # most debuggers harder to use. See "Getting to local determinism" and
config("compiler_defaults") {
   if (use_lld && host_os != "win") {
     ldflags += [ "-fuse-ld=lld" ]
   }
 
   if (llvm_build_instrumented_coverage) {
     cflags += [
       "-fcoverage-mapping",
 
+      # For build determinism. Using this requires passing --compilation-dir to
+      # llvm/utils/prepare-code-coverage-artifact.py.
+      "-fcoverage-compilation-dir=.",
+
       # Using an absolute path here is lame, but it's used at test execution
       # time to generate the profiles, and lit doesn't specify a fixed folder
       # for test execution -- so this is the only way to get all profiles into
       # a single folder like llvm/utils/prepare-code-coverage-artifact.py
       # expects.
       "-fprofile-instr-generate=" +
           rebase_path("$root_build_dir/profiles/%4m.profraw"),
     ]
-    if (use_goma) {
-      # goma has a bug where it writes the server-side absolute path.
-      # Work around that.
-      # FIXME: Instead, set this to `.` for deterministic builds and pass
-      # the build dir to prepare-code-coverage-artifact.py instead.
-      cflags += [ "-fcoverage-compilation-dir=" + rebase_path(root_build_dir) ]
-    }
-
     if (host_os != "win") {
       ldflags += [ "-fprofile-instr-generate" ]
     }
   }
 
   # Deterministic build setup, see
   # http://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html
   if (current_os == "win") {