Adds min-preludes to a few more slowest tests, and adds them to most of
the lowering tests, with a few exceptions that make use of operators.
This take the runtime of file_test down from about 8s to about 7s on my
machine.
We add support for Negate on uints in the min-preludes.
In LLVM, CLANG_DEFAULT_PIE_ON_LINUX is
[configurable](https://github.com/llvm/llvm-project/blob/main/clang/test/CMakeLists.txt#L8),
so change lowering to specify a value.
Also, adjust how the target is set so that function_decl.carbon isn't
trying to overload every flag, and so that the target isn't forgotten
elsewhere.
This is fixing issues introduced by #5427; note #5520 is also a related
fix.
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
This requires:
* Making `FunctionDecl` mutable since generating code
(`HandleTopLevelDecl()`) requires a mutable declaration and since we
manually add `used` attribute to force code generation.
* Passing the file system to `Lower` since it's needed by Clang code
generation.
* Creating an internal Clang LLVM module and link it against the Carbon
LLVM module.
Demo:
```c++
// hello_world.h
extern int puts;
inline void hello_world() {
((int (*)(const char*))&puts)("hello world");
}
```
```carbon
// main.carbon
library "Main";
import Cpp library "hello_world.h";
fn Run() -> i32 {
Cpp.hello_world();
return 0;
}
```
```shell
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link main.o --output=demo
$ ./demo
hello world
```
Based on https://github.com/carbon-language/carbon-lang/pull/5406.
Part of #5405.