Merge commit e09107ab8 from llvm git (by Raul Tambre):

Merge commit e09107ab8 from llvm git (by Raul Tambre):

  [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

This should fix 'Assertion failed: (i < getNumParams() && "Illegal
param #"), function getParamDecl, file
/usr/src/contrib/llvm-project/clang/include/clang/AST/Decl.h, line 2430'
when building the graphics/pgplot port.

Note that there may also have been other ports which triggered this
assertion, if they redeclare standard functions with incompatible
arguments.

Reported by:	zeising
MFC after:	6 weeks
X-MFC-With:	r364284