Bump blink_gc_plugin version number to trigger redownload of the clang package again

This should fix the iOS builder include errors as the previous Mac package failed
to include the include/ dir.

BUG=400088
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/586923002

Cr-Commit-Position: refs/heads/master@{#295798}

tools/clang/blink_gc_plugin/CMakeLists.txt

@@ -37,7 +37,7 @@ include_directories("${LLVM_SRC_DIR}/include"
 
 # This line is read by update.sh and other scripts in tools/clang/scripts
 # Note: The spaces are significant.
-set(LIBRARYNAME BlinkGCPlugin_11)
+set(LIBRARYNAME BlinkGCPlugin_12)
 
 add_llvm_loadable_module("lib${LIBRARYNAME}"
                          BlinkGCPlugin.cpp