First patch for #74368. Constant folding will be added in a follow-up
patch.

While pvalloc() is a legacy POSIX function, it remains widely available
in common C libraries like glibc.

Model pvalloc() in TargetLibraryInfo, allowing LLVM to correctly infer
its attributes.

This patch adds `nocallback` attributes for string/math libcalls. It
allows FuncAttributor to infer `norecurse` more precisely and encourages
more aggressive global optimization.

Reland https://github.com/llvm/llvm-project/pull/108985

Extend `LoopIdiomRecognize` to find and replace loops of the form
```c
base = str;
while (*str)
  ++str;
```
and transforming the `strlen` loop idiom into the appropriate `strlen`
and `wcslen` library call which will give a small performance boost if
replaced.
```c
str = base + strlen(base)
len = str - base
```

This reverts commit ac9049df7e62e2ca4dc5d103593b51639b5715e3.

This change broke Wine (causing it to hang on startup), see
https://github.com/llvm/llvm-project/pull/108985 for discussion.

Relands https://github.com/llvm/llvm-project/pull/108985

This PR continues the effort made in
https://discourse.llvm.org/t/rfc-strlen-loop-idiom-recognition-folding/55848
and https://reviews.llvm.org/D83392 and https://reviews.llvm.org/D88460
to extend `LoopIdiomRecognize` to find and replace loops of the form
```c
base = str;
while (*str)
  ++str;
```
and transforming the `strlen` loop idiom into the appropriate `strlen`
and `wcslen` library call which will give a small performance boost if
replaced.
```c
str = base + strlen(base)
len = str - base
```

This reverts commit bf6357f0f51eccc48b92a130afb51c0280d56180.

This PR continues the effort made in
https://discourse.llvm.org/t/rfc-strlen-loop-idiom-recognition-folding/55848
and https://reviews.llvm.org/D83392 and https://reviews.llvm.org/D88460
to extend `LoopIdiomRecognize` to find and replace loops of the form
```c
base = str;
while (*str)
  ++str;
```
and transforming the `strlen` loop idiom into the appropriate `strlen`
and `wcslen` library call which will give a small performance boost if
replaced.
```c
str = base + strlen(base)
len = str - base
```

---------

Co-authored-by: Michael Kruse <github@meinersbur.de>

Attributes inference has been improved for a few functions.
Particularly, ldexp and variants, as well as abort, may be
marked as `nounwind`, as they do not propagate any exceptions
to the caller, neither they unwind the stack. Besides, fwrite
and fread first argument should be respectively readonly and
writeonly.

Floating-point libcalls are currently conservatively marked as
may write any memory. Restrict these to clobber only `errno`.