For assigning SourceLocations to macro arg tokens, reserve a single SLocEntry

For assigning SourceLocations to macro arg tokens, reserve a single SLocEntry
for tokens that are lexed consecutively from the same FileID, instead of creating
a SLocEntry for each token. e.g for

   assert(foo == bar);

there will be a single SLocEntry for the "foo == bar" chunk and locations
for the 'foo', '==', 'bar' tokens will point inside that chunk.

For parsing SemaExpr.cpp, this reduced the number of SLocEntries by 25%.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138129 91177308-0d34-0410-b5e6-96231b3b80d8