Sanitizers: explicitly disable use_sanitizer_coverage when use_clang_coverage=true.

A follow-up for https://chromium-review.googlesource.com/c/chromium/src/+/1112199.
These two changes should significantly speed up code coverage builds. Build time,
binary sizes and runtime all get improved by ~2x. See the other CL description and
the bugs referenced for more precise numbers and targets tested.

We already have an assertion against enabling both use_sanitizer_coverage and
use_clang_coverage via GN args, but the former still could be turned on indirectly
if a fuzzing engine is used (e.g. use_libfuzzer or use_afl).

Bug: 846054
Change-Id: Idcd87a4589eda71e519d382ef235b38cc0984ed3
Reviewed-on: https://chromium-review.googlesource.com/1112305Reviewed-by: Abhishek Arya <inferno@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569779}

build/config/sanitizers/sanitizers.gni

@@ -4,6 +4,7 @@
 
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
+import("//build/config/coverage/coverage.gni")
 import("//build/toolchain/toolchain.gni")
 
 declare_args() {
@@ -146,7 +147,9 @@ use_fuzzing_engine = use_libfuzzer || use_afl
 # declare_args block. User overrides are only applied at the end of a
 # declare_args block.
 declare_args() {
-  use_sanitizer_coverage = use_fuzzing_engine || sanitizer_coverage_flags != ""
+  use_sanitizer_coverage =
+      !use_clang_coverage &&
+      (use_fuzzing_engine || sanitizer_coverage_flags != "")
 
   # Detect overflow/underflow for global objects.
   #