Build: disable ICF when using clang_code_coverage=true.

When ICF (Identical Code Folding) is enabled, the linker has hard time figuring
out all identical code patterns inserted by compiler's instrumentation. That is
why this option is disabled when we use sanitizers. It also should be disabled
for code coverage instrumented build, as it has even more instrumentation code.

This change allows code coverage bots to use use_sanitizer_coverage=false which
would result in much faster and much smaller builds.

Performance improvements:
- builds 2x faster
- binaries 2x smaller
- run time 2x smaller (or even more on some targets, up to 6x)

Bug: 851337
Change-Id: I20d4ad1e55730daf813e0f26060dd5830351dbcf
Reviewed-on: https://chromium-review.googlesource.com/1112199
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Abhishek Arya <inferno@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569693}

build/config/compiler/BUILD.gn

@@ -149,7 +149,7 @@ declare_args() {
   #
   # lld doesn't have the bug.
   use_icf = (is_posix || is_fuchsia) && !using_sanitizer &&
-            !(is_android && use_order_profiling) &&
+            !use_clang_coverage && !(is_android && use_order_profiling) &&
             (use_lld ||
              (use_gold &&
               ((!is_android && linux_use_bundled_binutils) || is_chromeos ||