[flang] Faster implementation of FindScope()

The utility semantics::SemanticsContext::FindScope() maps a contiguous
range of cooked source characters to the innermost Scope containing
them.  Its implementation is unacceptably slow on large (tens of
thousands of lines) source files with many program units; it traverses
each level of the scope tree linearly.

Replace this implementation with a single instance of std::multimap<>
used as an index from each Scope's source range back to the Scope.

Compilation time with "-fsyntax-only" on the 50,000-line test case
that motivated this change drops from 4.36s to 3.72s, and FindScope()
no longer stands out egregiously in the profile.

Differential Revision: https://reviews.llvm.org/D159027