[ios] Remove support for building with Xcode clang

Building with Xcode clang (by setting `use_xcode_clang = true`)
is not maintained and only used to build with bitcode enabled.
As ToT clang supports building with -fembed-bitcode (or the
debug variable -fembed-bitcode-marker), there is no point in
keeping the support for building with Xcode clang.

The variable `use_xcode_clang` is still referenced from some
projects build on top of //build and included in Chromium
(such as Angle), so keep a definition of the variable until
those projects have been converted.

Simplify the BUILD.gn variables by applying the following two
transformations after removing the variable from the conditions:
-   `is_ios || is_mac` => `is_apple`
-   `if (a) { if (b) { ... } }` => `if (a && b) { ... }`

Bug: 946629, 1266466
Change-Id: I9f57b357375f73f38c62e63858ea63263f1dec93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3259660
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#939783}
NOKEYCHECK=True
GitOrigin-RevId: 6edcf847d80f13740e1a26a86d3b95a0bb9fbcad
diff --git a/BUILD.gn b/BUILD.gn
index 8d7097a..2b29672 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -142,7 +142,7 @@
       "-Wno-sign-conversion",
       "-Wstring-conversion",
     ]
-    if (!is_nacl && !use_xcode_clang) {
+    if (!is_nacl) {
       cflags_cc += [ "-Wbitfield-enum-conversion" ]
     }
   }