llvm-cov: Use ErrorOr rather than an error_code* (NFC)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217404 91177308-0d34-0410-b5e6-96231b3b80d8
Fix type error in insertvalue example in LangRef. %agg1 is of type {i32,
float} and thus cannot be used where a type {i32, {float}} is expected.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217405 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Try to appease MSVC after r217404

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217406 91177308-0d34-0410-b5e6-96231b3b80d8