The utility expressions in the `InstrumentationRuntime` plugins are just
plain C code, but we run them as `ObjC++`. That meant we were doing
redundant work (like looking up decls in the Objective-C runtime). The
sanitizer tests sporadically time out while looking up function symbols
in the Objective-C runtime. This patch switches the expression language
to `C`.
Didn't find a great way of testing this other than looking at the
expression log.
rdar://165656320
d7cbc7f9e4