Force inlining of Array.withUnsafeMutableBufferPointer
Force inlining of Array.withUnsafeMutableBufferPointer

We do this to enable removal of the closure (allocation).

The size of the libSwiftCore.dylib grows by 300 bytes. A program (such as
benchmark/single-source/unit-tests/StackPromo.swift) that uses one invocation of
withUnsafeMutableBufferPointer shrinks by roughly 500 bytes.

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