Fix overflow when searching for #>

[bitjammer]

ede6bf7 increments the buffer pointer to early
when searching for the <# prefix to stop lexing an operator,
so the source buffer is accessed 1 byte off the end.

Thanks ASan and thanks @gparker42!

rdar://problem/28457876

[bitjammer]

Doing an ASan run locally.

[bitjammer]

I knew there was a reason my body woke me up early today ...

[bitjammer]

@swift-ci Please smoke test

[gparker42]

rangeContainsPlaceholderEnd() looks good.

Is the call to isEditorPlaceholder() correct? I think it blows up if the buffer ends with <.

[bitjammer]

Oh, hm. So that might also go past the buffer by one?

[bitjammer]

Since isEditorPlaceHolder is just a startswith call, we could do this instead there, yeah?

Identifier::isEditorPlaceholder(StringRef(CurPtr, BufferEnd-CurPtr))

[bitjammer]

@swift-ci Please smoke test

[bitjammer]

@swift-ci Please smoke test f'real

[bitjammer]

@swift-ci Pretty please smoke test

[gparker42]

BufferEnd-CurPtr looks safe. We already know that CurPtr is not past BufferEnd.

[bitjammer]

@swift-ci Please asan test

[bitjammer]

The ASan test is running but not showing up here: https://ci.swift.org/job/swift-PR-osx-ASAN-test/46

[gparker42]

ASAN test succeeded. This should be good to go.

[bitjammer]

Thanks again @gparker42 !