[Clang] Ensure the method scope at the late parsing of noexcept specifiers (#98023)
Previously, we only pushed the function scope once we entered the
function definition, whereas tryCaptureVariable() requires at least one
function scope available when ParmVarDecls being captured have been
owned by a function. This led to problems parsing the noexcept
specifiers, as the DeclRefExprs inside them were improperly computed.
Fixes https://github.com/llvm/llvm-project/issues/97453