[Clang] Refactor `__attribute__((assume))` (#84934)

This is a followup to #81014 and #84582: Before this patch, Clang
would accept `__attribute__((assume))` and `[[clang::assume]]` as
nonstandar

[Clang] Refactor `__attribute__((assume))` (#84934)

This is a followup to #81014 and #84582: Before this patch, Clang
would accept `__attribute__((assume))` and `[[clang::assume]]` as
nonstandard spellings for the `[[omp::assume]]` attribute; this
resulted in a potentially very confusing name clash with C++23’s
`[[assume]]` attribute (and GCC’s `assume` attribute with the same
semantics).

This pr replaces every usage of `__attribute__((assume))` with
`[[omp::assume]]` and makes `__attribute__((assume))` and
`[[clang::assume]]` alternative spellings for C++23’s `[[assume]]`;
this shouldn’t cause any problems due to differences in appertainment
and because almost no-one was using this variant spelling to begin
with (a use in libclc has already been changed to use a different
attribute).

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[OpenMP][Docs] Make copy pasting remarks easier

[OpenMP] Eliminate redundant barriers in the same block

Patch originally by Giorgis Georgakoudis (@ggeorgakoudis), typos and
bugs introduced later by me.

This patch allows us to remove redundant ba

[OpenMP] Eliminate redundant barriers in the same block

Patch originally by Giorgis Georgakoudis (@ggeorgakoudis), typos and
bugs introduced later by me.

This patch allows us to remove redundant barriers if they are part
of a "consecutive" pair of barriers in a basic block with no impacted
memory effect (read or write) in-between them. Memory accesses to
local (=thread private) or constant memory are allowed to appear.
Technically we could also allow any other memory that is not used to
share information between threads, e.g., the result of a malloc that
is also not captured. However, it will be easier to do more reasoning
once the code is put into an AA. That will also allow us to look through
phis/selects reasonably. At that point we should also deal with calls,
barriers in different blocks, and other complexities.

Differential Revision: https://reviews.llvm.org/D118002

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[OpenMP] Add more verbose remarks for runtime folding

We peform runtime folding, but do not currently emit remarks when it is
performed. This is because it comes from the runtime library and is
beyo

[OpenMP] Add more verbose remarks for runtime folding

We peform runtime folding, but do not currently emit remarks when it is
performed. This is because it comes from the runtime library and is
beyond the users control. However, people may still wish to view this
and similar information easily, so we can enable this behaviour using a
special flag to enable verbose remarks.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D109627

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[OpenMP] Add remark documentation to the OpenMP webpage

This patch begins adding documentation for each remark emitted by
`openmp-opt`. This builds on the IDs introduced in D105939 so that users
can

[OpenMP] Add remark documentation to the OpenMP webpage

This patch begins adding documentation for each remark emitted by
`openmp-opt`. This builds on the IDs introduced in D105939 so that users
can more easily identify each remark in the webpage.

Depends on D105939.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106018

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[OpenMP][Docs] Add remarks intro section

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D93735

[OpenMP][NFC] Provide a new remark and documentation

If a GPU function is externally reachable we give up trying to find the
(unique) kernel it is called from. This can hinder optimizations. Emit a

[OpenMP][NFC] Provide a new remark and documentation

If a GPU function is externally reachable we give up trying to find the
(unique) kernel it is called from. This can hinder optimizations. Emit a
remark and explain mitigation strategies.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D93439

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[OpenMP][Docs] Structure and content for the OpenMP documentation

This adds some initial content as well as structure to the new OpenMP
Sphinx documentation hosted at http://openmp.llvm.org/docs/ .

[OpenMP][Docs] Structure and content for the OpenMP documentation

This adds some initial content as well as structure to the new OpenMP
Sphinx documentation hosted at http://openmp.llvm.org/docs/ .

The content contains some useful links but most pages are still empty.

This uses a "custom" theme which is a copy of the default "agogo" one
with minor modifications to get a nicer table of content in the sidebar.
This way we can also adjust the theme as we go.

Reviewed By: jhuber6, JonChesterfield

Differential Revision: https://reviews.llvm.org/D90256

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