Replace some typically large vectors with SmallVector.

This may seem counter-intuitive but the POD-like optimization helps when the
vectors grow into multimegabyte buffers. SmallVector calls realloc which knows
how to twiddle virtual memory bits and avoids large copies.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175906 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/SourceManager.cpp b/lib/Basic/SourceManager.cpp
index 901d3a6..c5d275a 100644
--- a/lib/Basic/SourceManager.cpp
+++ b/lib/Basic/SourceManager.cpp
@@ -721,7 +721,7 @@
 
   // See if this is near the file point - worst case we start scanning from the
   // most newly created FileID.
-  std::vector<SrcMgr::SLocEntry>::const_iterator I;
+  const SrcMgr::SLocEntry *I;
 
   if (LastFileIDLookup.ID < 0 ||
       LocalSLocEntryTable[LastFileIDLookup.ID].getOffset() < SLocOffset) {