Fix SourceMgrDiagnosticHandler::convertLocToSMLoc for unknown line/column scenerio.

Summary: FileLineColLoc allows the column and line to be zero to represent unknown column and/or unknown line/column information. However, SourceMgr::FindLocForLineAndColumn treats line 0 and col 0 valid and pointing to the first line and col, respectively. To adapt this mismatch in semantics, we explicitly check line/col being zeros in SourceMgrDiagnosticHandler::convertLocToSMLoc

Differential Revision: https://reviews.llvm.org/D80258