Clang: Use completion through backend process

This makes us independent of libclang crashes for completion.
Re-parsing for highlighting still happens in the Qt Creator process.

Run in verbose mode:
    qtc.clangcodemodel.ipc=true

Run tests:
    -test "ClangCodeModel"

Task-number: QTCREATORBUG-14108
Task-number: QTCREATORBUG-12819
Change-Id: Id3e95bd2afdb6508bbd1d35fddc69534a909b905
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>

src/plugins/clangcodemodel/constants.h

@@ -55,6 +55,8 @@ static const QLatin1Char kDoubleQuote('"');
 static const QLatin1Char kNewLine('\n');
 static const QLatin1Char kHorizontalTab('\t');
 
+const char CLANG_MODELMANAGERSUPPORT_ID[] = "ClangCodeMode.ClangCodeMode";
+
 }
 }