Cherry-pick of #80547 for the 6.2 release branch.
---
Explanation: This cherry picks the implementation of SE-0477 to add a
string interpolation method with a `default:` parameter for optional
interpolation values.
Main Branch PR: https://github.com/swiftlang/swift/pull/80547
Risk: Low.
Reviewed By: @stephentyrone
Resolves: rdar://150865613
Testing: New tests for the string interpolations and fix-its.
`String.debugDescription` currently fails to protect the contents of
the string from combining with the opening or closing `”` characters
or one of the characters of a quoted scalar:
```swift
let s = “\u{301}A\n\u{302}B\u{70F}”
print(s.debugDescription)
// ⟹ “́A\n̂B” (characters: “́, A, \, n̂, B, ”)
```
This can make debug output difficult to read, as string contents are
allowed to spread over and pollute neighboring meta-characters.
This change fixes this by force-quoting the problematic scalars in
these cases:
```swift
let s = “\u{301}A\n\u{302}B\u{70F}”
print(s.debugDescription)
// ⟹ “\u{301}A\n\u{302}B\u{70F}”
```
Of course, Unicode scalars that don’t engage in such behavior are
still allowed to pass through unchanged:
```swift
let s = “Cafe\u{301}”
print(s.debugDescription)
// ⟹ “Café”
```
The `-force-single-frontend-invocation` flag predates WMO and is now an
alias for `-whole-module-optimization`. We should use the latter and let
the former fade into history.
