[clang][Syntax] Optimize expandedTokens for token ranges.

[clang][Syntax] Optimize expandedTokens for token ranges.

`expandedTokens(SourceRange)` used to do a binary search to get the
expanded tokens belonging to a source range. Each binary search uses
`isBeforeInTranslationUnit` to order two source locations. This is
inherently very slow.
By profiling clangd we found out that users like clangd::SelectionTree
spend 95% of time in `isBeforeInTranslationUnit`. Also it is worth
noting that users of `expandedTokens(SourceRange)` majorly use ranges
provided by AST to query this funciton. The ranges provided by AST are
token ranges (starting at the beginning of a token and ending at the
beginning of another token).

Therefore we can avoid the binary search in majority of the cases by
maintaining an index of ExpandedToken by their SourceLocations. We still
do binary search for ranges which are not token ranges but such
instances are quite low.

Performance:
`~/build/bin/clangd --check=clang/lib/Serialization/ASTReader.cpp`
Before: Took 2:10s to complete.
Now: Took 1:13s to complete.

Differential Revision: https://reviews.llvm.org/D99086