Isolate BoringSSL for iOS builds (#26109)

Prefix BoringSSL APIs with _GRPC_ by default on iOS, by actually isolating it against other libraries with might include openssl themselves and therefore allowing to embed gRPC on iOS completely mind free.

This patch essentially does what has already been done for cocoapods ( see https://github.com/grpc/grpc/pull/21194 ).

src/csharp/experimental/build_native_ext_for_ios.sh

@@ -28,7 +28,7 @@ function build {
     PATH_CC="$(xcrun --sdk $SDK --find clang)"
     PATH_CXX="$(xcrun --sdk $SDK --find clang++)"
 
-    CPPFLAGS="-O2 -Wframe-larger-than=16384 -arch $ARCH -isysroot $(xcrun --sdk $SDK --show-sdk-path) -mios-version-min=9.0 -DPB_NO_PACKED_STRUCTS=1"
+    CPPFLAGS="-O2 -Wframe-larger-than=16384 -arch $ARCH -isysroot $(xcrun --sdk $SDK --show-sdk-path) -mios-version-min=9.0 -DPB_NO_PACKED_STRUCTS=1 -DBORINGSSL_PREFIX=GRPC -Isrc/boringssl"
     LDFLAGS="-arch $ARCH -isysroot $(xcrun --sdk $SDK --show-sdk-path) -Wl,ios_version_min=9.0"
 
     # TODO(jtattermusch): Ideally we'd be setting build defines that correspond to using cmake's