[clang] Casting unsigned long _Fract to int emits out-of-bounds shift

Casting `unsigned long _Fract` to an integer emits an out-of-bounds shift by the bitwidth in `FixedPointBuilder`, leading to undefined behavior in LLVM IR.

When downscaling in `FixedPointBuilder::Convert` (`llvm/include/llvm/IR/FixedPointBuilder.h`), it creates a right shift by `SrcScale - DstScale`:

```cpp
unsigned Shift = SrcScale - DstScale;
if (SrcIsSigned)
Result = B.CreateAShr(Result, Shift, "downscale");
else
Result = B.CreateLShr(Result, Shift, "downscale");

```

For `unsigned long _Fract`, `SrcScale` is 32 (its full bitwidth) and `DstScale` is 0. This emits `lshr i32 %x, 32`, which is an invalid shift by the operand bitwidth.

For example:

```cpp
int to_int(unsigned long _Fract x) {
return (int)x;
}
```

Compiling with `clang++ -ffixed-point -O0 -S -emit-llvm` gives:

```llvm
define dso_local noundef i32 @_Z6to_intDRm(i32 noundef %0) {
...
%3 = load i32, ptr %2, align 4
%4 = lshr i32 %3, 32
ret i32 %4
}
```

Since `unsigned long _Fract` values are in [0, 1), converting to an integer should evaluate to 0.

This was uncovered by a test failure in LLVM-libc's `countlsulr_test` (`libc/test/src/stdfix/CountlsTest.h`). In `CountlsTest.h`, the check `if (10 <= static_cast(max))` became undefined due to the invalid shift. At runtime the backend branched on uninitialized register state, which occasionally took the branch and failed with `Expected: FXRep::INTEGRAL_LEN - 4 (-4), To be equal to: func(10) (0)`.

In `FixedPointBuilder::Convert`, if `SrcScale - DstScale >= ResultWidth`, downscaling an unsigned value should return constant zero instead of emitting an invalid shift.

Assisted-by: Automated tooling, human reviewed

Start in llvm/include/llvm/IR/FixedPointBuilder.h, focusing on FixedPointBuilder::Convert and its downscaling shift logic. Then inspect llvm/libc/test/src/stdfix/CountlsTest.h and run the relevant countlsulr_test. Done means converting unsigned long _Fract values such as 10 produces defined LLVM IR and the test no longer observes an invalid shift or incorrect result.

Written by the indexing model from the issue text.