Add Natvis visualiser and debuginfo tests for f128

[beetrees]

To render f128s in debuggers on MSVC targets, this PR changes the compiler to output f128s as struct f128 { low_bits: u64, high_bits: u64 }, and includes a Natvis visualiser that displays the float in hexdecimal format (similar to the LowerHex impl in #160626), as unlike f16 there's no larger supported float format to convert to and trying to write a float to decimal string converter in Natvis didn't seem practical. gdb, lldb and cdb tests are also included for f128, although gdb does not yet correctly identify the float format.

I did consider also displaying a double approximation of the f128 (as the debugger will convert that to a decimal string) but decided against it as f64 has a significantly smaller exponent range too (meaning very large or small f128s couldn't have an approximation anyway) and cdb and WinDbg both seem to round all floating-point numbers to 6 decimal places, meaning the displayed number is (double) rounded even further than f64 precision and is useless for tiny values.

Closes #121837
Tracking issue: #116909

[rustbot]

r? @JohnTitor

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[beetrees]

cc MSVC target maintainers: @ChrisDenton @dpaoliello @Fulgen301 @lambdageek @sivadeilra @wesleywiser

[beetrees]

(Given this touches debuginfo tests this should probably get a bors try x86_64-msvc-* i686-msvc-* aarch64-msvc-* aarch64-apple-*.)

[folkertdev]

@bors try jobs=x86_64-msvc-,i686-msvc-,aarch64-msvc-,aarch64-apple-

[folkertdev]

@bors try jobs=x86_64-msvc-,i686-msvc-,aarch64-msvc-,aarch64-apple-

[rust-bors]

💔 Test for 816a2f0 failed: CI. Failed jobs:

• try - aarch64-apple-1 (web logs, enhanced plaintext logs)
• try - aarch64-apple-macos-26-1 (web logs, enhanced plaintext logs)

[beetrees]

According to the table on Wikipedia, there isn't any Xcode versions yet with LLDB 22 or later, so I've ignored the f128 debuginfo tests on Apple for now and left a FIXME.

[folkertdev]

@bors try jobs=x86_64-msvc-,i686-msvc-,aarch64-msvc-,aarch64-apple-

[rust-bors]

☀️ Try build successful (CI)
Build commit: d79cce8 (d79cce855e6f305d9b8796f97b79e09fb0a5c022)
Base parent: 3ffb26f (3ffb26fbf5bf232cf59e314e75ea325973f4f583)

[beetrees]

@rustbot review

[JohnTitor]

Looks good! I'm not going to r+ just in case you want to wait for another look from the target maintainers. Ohterwise r=me

View changes since this review

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[beetrees]

I've updated the LLDB tests that this PR adds in basic-types-mut-globals.rs to match the changes made to the existing ones in basic-types-globals.rs in the rebase. Not sure if this counts as significant enough to require a review or if the r=me still stands, so I'll air on the side of caution.