Remove StringMap::GetOrCreateValue in favor of StringMap::insert

Having two ways to do this doesn't seem terribly helpful and
consistently using the insert version (which we already has) seems like
it'll make the code easier to understand to anyone working with standard
data structures. (I also updated many references to the Entry's
key and value to use first() and second instead of getKey{Data,Length,}
and get/setValue - for similar consistency)

Also removes the GetOrCreateValue functions so there's less surface area
to StringMap to fix/improve/change/accommodate move semantics, etc.

llvm-svn: 222319
diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index 220747d..5effb01 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -368,10 +368,10 @@
     FileNumber = SourceIdMap.size() + 1;
     assert((MCDwarfFiles.empty() || FileNumber == MCDwarfFiles.size()) &&
            "Don't mix autonumbered and explicit numbered line table usage");
-    StringMapEntry<unsigned> &Ent = SourceIdMap.GetOrCreateValue(
-        (Directory + Twine('\0') + FileName).str(), FileNumber);
-    if (Ent.getValue() != FileNumber)
-      return Ent.getValue();
+    auto IterBool = SourceIdMap.insert(
+        std::make_pair((Directory + Twine('\0') + FileName).str(), FileNumber));
+    if (!IterBool.second)
+      return IterBool.first->second;
   }
   // Make space for this FileNumber in the MCDwarfFiles vector if needed.
   MCDwarfFiles.resize(FileNumber + 1);