If `__attribute__((flatten))` is used on a function, or
`[[clang::always_inline]]` on a statement, don't inline any callees with
incompatible streaming attributes. Without this check, clang may produce
incorrect code when these attributes are used in code with streaming
functions.
Note: The docs for flatten say it can be ignored when inlining is
impossible: "causes calls within the attributed function to be inlined
unless it is impossible to do so".
Similarly, the (clang-only) `[[clang::always_inline]]` statement
attribute is more relaxed than the GNU `__attribute__((always_inline))`
(which says it should error it if it can't inline), saying only "If a
statement is marked [[clang::always_inline]] and contains calls, the
compiler attempts to inline those calls.". The docs also go on to show
an example of where `[[clang::always_inline]]` has no effect.
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