Substring Float parsing incorrectly considers the entire base string

[Lukasa]

Description

It looks like #85797 has broken Float parsing of Substrings. Specifically, the Float parsing now appears to consider the whole base string, not just the relevant piece of the Substring.

Reproduction

func main() {
    print(String(describing: Float("0.3")))
    print(String(describing: Float("00.3".dropFirst())))
    print(String(describing: Float("=0.3".dropFirst())))
    print(String(describing: Float("0.35".dropLast())))
}

main()

Build and run from the file, e.g. swift test.swift.

This produces:

Optional(0.3)
Optional(0.3)
nil
Optional(0.35)

Observe that the third case fails to parse, even though the = was dropped from the Substring, and the latter case incorrectly parses as 0.35 even though the 5 was dropped as well.

Expected behavior

This should print:

Optional(0.3)
Optional(0.3)
Optional(0.3)
Optional(0.3)

Environment

Swift version 6.3-dev (LLVM cff341857efbfc7, Swift 956dc0c1a258891)
Target: aarch64-unknown-linux-gnu
Build config: +assertions

From latest nightly docker image as of today.

Additional information

No response

[Lukasa]

It looks like #86018 should fix this.