constify BeforeThanCompare<SourceRange>::operator()

[Graham Lee]

Hi, 
 
this patch simply adds the const qualifier to the operator() function in SourceManager.h. The qualifier is present for the SourceLocation version of the comparator but not the SourceRange version. No build problems were introduced by doing this, and it solved a problem I had in building my own tool :). 
 
Thanks, 
Graham. 
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