SourceKit uses a clever pragma to automatically attach an attribute to
every declaration in the file. However, the form of this pragma (clang
attribute push) uses a parenthesized list marked with 'any', which turns
out to be a name that people sometimes use for macros. Guard against this
using the push_macro pragma (originally from MSVC), under the assumption
that anyone who supports the highly-use-specific 'external_source_symbol'
attribute probably implements this more common extension. This does make
the generated header a little uglier, but it's not like it was really
pretty to begin with.
rdar://problem/34168022
