AddressFunctionScope was always returning the first address range of the
function (assuming it was the only one). This doesn't work for
RegisterContextUnwind (it's only caller), when the function doesn't
start at the lowest address because it throws off the 'how many bytes
"into" a function I am' computation. This patch replaces the result with
a call to (recently introduced)
SymbolContext::GetFunctionOrSymbolAddress.
85c3c98630