[lldb][Format] Add function.suffix frame-format variable (#137763)
This patch adds another frame-format variable (currently only
implemented in the CPlusPlus language plugin) that represents the
"suffix" of a function. The name is derived from the `DotSuffix` node of
LLVM's Itanium demangler.
For a function name such as `int foo() (.cold)`, the suffix would be
`(.cold)`.