Before this patch, the use of GenericSignature::getSubstitutionMap to
create a substitution map would result in some generic parameters being
completely elided (well, being mapped to concrete types causing a
->castTo<GenericTypeParamType>() to crash). This patch changes that to
just do the reindexing of the type parameters directly, without trying
to be smart with more contextual information (i.e. τ_0_0 -> τ_1_0, even
if there's a τ_0_0 == Int requirement).
Fixes rdar://problem/37291254 and SR-6990.
Move the rest of associated type inference into the new source file,
and split RequirementEnvironment into its own AST-level header, because it
can be re-used and has no ties to the type checker.
