[libc++][NFC] Add missing license headers

Also standardize the license comment in several files where it was
different from what we normally do.

[libc++][format] Improves diagnostics.

Improves both the compile-time and run-time errors.
At compile-time it does a bit more work to get more specific errors.
This could be done at run-time too, bu

[libc++][format] Improves diagnostics.

Improves both the compile-time and run-time errors.
At compile-time it does a bit more work to get more specific errors.
This could be done at run-time too, but that has a performance penalty.
Since it's expected most use-cases use format* instead of vformat* the
compile-time errors are more common.

For example when using

std::format_to("{:-c}", 42);

Before compile output would contain

std::__throw_format_error("The format-spec should consume the input or end with a '}'");

Now it contains

std::__throw_format_error("The format specifier does not allow the sign option");

Given a better indication the sign option is not allowed. Note the
output is still not user-friendly; C++ doesn't have good facilities to
generate nice messages from the library.

In general all messages have been reviewed and improved, using a more
consistent style and using less terms used in the standard. For example

format-spec -> format specifier
arg-id -> argument index

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D152624

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[libc++][format] Improves run-time diagnostics.

After parsing a std-format-spec it's validated, depending on the type used some
format options are not allowed. This improves the error messages in th

[libc++][format] Improves run-time diagnostics.

After parsing a std-format-spec it's validated, depending on the type used some
format options are not allowed. This improves the error messages in the
exceptions thrown upon failure.

Depends on D155364

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D155366

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[libc++][print] Adds FILE functions.

Drive-by fix to make sure the __retarget_buffer works correctly whan
using a hint of 1. This was discovered in one of the new tests.

Drive-by fixes __retarget_b

[libc++][print] Adds FILE functions.

Drive-by fix to make sure the __retarget_buffer works correctly whan
using a hint of 1. This was discovered in one of the new tests.

Drive-by fixes __retarget_buffer when initialized with size 1.

Implements parts of
- P2093R14 Formatted output
- P2539R4 Should the output of std::print to a terminal be
synchronized with the underlying stream?

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D150044

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[libc++] Mark slow tests as unsupported on GCC

Some tests in our test suite are unbelievably slow on GCC due to the
use of the always_inline attribute. See [1] for more details.

This patch introduc

[libc++] Mark slow tests as unsupported on GCC

Some tests in our test suite are unbelievably slow on GCC due to the
use of the always_inline attribute. See [1] for more details.

This patch introduces the GCC-ALWAYS_INLINE-FIXME lit feature to
disable tests that are plagued by that issue. At the same time, it
moves several existing tests from ad-hoc `UNSUPPORTED: gcc-12` markup
to the new GCC-ALWAYS_INLINE-FIXME feature, and marks the slowest tests
reported by the CI as `UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME`.

[1]: https://discourse.llvm.org/t/rfc-stop-supporting-extern-instantiations-with-gcc/71277/1

Differential Revision: https://reviews.llvm.org/D152736

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[libc++][format] Improves fill character.

The main change is to allow a UCS scalar value as fill character.
Especially for char based formatting this increase the number of valid
characters. Origina

[libc++][format] Improves fill character.

The main change is to allow a UCS scalar value as fill character.
Especially for char based formatting this increase the number of valid
characters. Originally this was to be expected ABI breaking, however the
current change does not seem to break the ABI.

Implements
- P2572 std::format() fill character allowances

Depends on D144499

Reviewed By: ldionne, tahonermann, #libc

Differential Revision: https://reviews.llvm.org/D144742

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