Clang: Use MessageEnvelop instead of QVariant

QVariant has unwanted dependencies so we provided our own simpler solution.
We want to support move only types and calling the copy constructor as you
move the value in and outside. This copying is adding unwanted overhead
too.

Change-Id: I2e27a7924868efe81e8b8ff3415499c9fa22c2bc
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/clangbackendipc/projectpartcontainer.cpp

@@ -73,11 +73,6 @@ bool operator==(const ProjectPartContainer &first, const ProjectPartContainer &s
     return first.projectPartId_ == second.projectPartId_;
 }
 
-bool operator<(const ProjectPartContainer &first, const ProjectPartContainer &second)
-{
-    return first.projectPartId_ < second.projectPartId_;
-}
-
 static Utf8String quotedArguments(const Utf8StringVector &arguments)
 {
     const Utf8String quote = Utf8String::fromUtf8("\"");