[Darwin] Export new weak external symbols when compiling with coverage
[Darwin] Export new weak external symbols when compiling with coverage

Some weak external symbols were added to the profile runtime in D49953,
and on Darwin, these need to be exported for tapi verification purposes.

I've tightened the test so that future breakages can be caught earlier.

rdar://45831054

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@346276 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt