[Parse] Create DiagnosticTransaction along with BacktrackingScope

[rintaro]

to prevent duplicated diagnostics.

For instance, on Int8(exactly: 0xG), the following error was emitted two times:

test.swift:1:21: error: expected a digit after integer literal prefix
_ = Int8(exactly: 0xG)
                    ^
test.swift:1:21: error: expected a digit after integer literal prefix
_ = Int8(exactly: 0xG)
                    ^

In this change,
DiagnosticTransaction is committed on BacktrackingScope::cancelBacktrack(),
or aborted during BacktrackingScope destruction.

[rintaro]

@swift-ci Please smoke test

[rintaro]

This PR will affect the testcase of #5270.
Either one should be merged first.
CC: @DougGregor @CodaFi

[CodaFi]

Could you merge this one first please? I already have to edit the diagnostics in #5270 anyway.

[DougGregor]

LGTM! I'll go ahead and merge so @CodaFi can land his change.