Fix incorrect Twine usage in CFGPrinter

CFGPrinter (-view-cfg, -dot-cfg) invokes an undefined behaviour (dangling
pointer to rvalue) on IR files with branch weights. This patch fixes the
problem caused by Twine initialization and string conversion split into
two statements.

This change fixes the bug 37019. A similar patch to this problem was
provided in the llvmlite project

Patch by mcopik (Marcin Copik).

Differential Revision: https://reviews.llvm.org/D52933

llvm-svn: 343984