[Sema] Introduce BuiltinAttr, per-declaration builtin-ness

Instead of relying on whether a certain identifier is a builtin,
introduce BuiltinAttr to specify a declaration as having builtin
semantics.

This fixes incompatible redeclarations of builtins, as reverting the
identifier as being builtin due to one incompatible redeclaration
would have broken rest of the builtin calls.
Mostly-compatible redeclarations of builtins also no longer have
builtin semantics. They don't call the builtin nor inherit their
attributes.
A long-standing FIXME regarding builtins inside a namespace enclosed
in extern "C" not being recognized is also addressed.

Due to the more correct handling attributes for builtin functions are
added in more places, resulting in more useful warnings.
Tests are updated to reflect that.

Intrinsics without an inline definition in intrin.h had `inline` and
`static` removed as they had no effect and caused them to no longer
be recognized as builtins otherwise.

A pthread_create() related test is XFAIL-ed, as it relied on it being
recognized as a builtin based on its name.
The builtin declaration syntax is too restrictive and doesn't allow
custom structs, function pointers, etc.
It seems to be the only case and fixing this would require reworking
the current builtin syntax, so this seems acceptable.

Fixes PR45410.

Reviewed By: rsmith, yutsumi

Differential Revision: https://reviews.llvm.org/D77491