DebugInfo: Rewrite llvm::getDISubprogram(), NFC

Simplify implementation of `llvm::getDISubprogram()`.  I might go
through and see how difficult it is to update the users, since this
function doesn't really seem necessary anymore.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233662 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed