[Exclusivity] Relax enforcement for separate struct stored properties

Relax the static enforcement of exclusive access so that we no longer diagnose
on accesses to separate stored structs of the same property:

takesInout(&s.f1, &s.f2) // no-warning

And perform the analogous relaxation for tuple elements.

To do this, track for each begin_access the projection path from that
access and record the read and write-like modifications on a per-subpath
basis.

We still warn if the there are conflicting accesses on subpaths where one is
the prefix of another.

This commit leaves the diagnostic text in a not-so-good shape since we refer
to the DescriptiveDeclKind of the access even describing a subpath.

I'll fix that up in a later commit that changes only diagnostic text.

https://bugs.swift.org/browse/SR-5119
rdar://problem/31909639

Author: devincoughlin

include/swift/AST/DiagnosticsSIL.def

@@ -93,13 +93,13 @@ WARNING(exclusivity_access_required_swift3,none,
         "simultaneous accesses to %0 %1, but "
         "%select{initialization|read|modification|deinitialization}2 requires "
         "exclusive access; consider copying to a local variable",
-        (DescriptiveDeclKind, Identifier, unsigned))
+        (DescriptiveDeclKind, StringRef, unsigned))
 
 ERROR(exclusivity_access_required,none,
       "simultaneous accesses to %0 %1, but "
       "%select{initialization|read|modification|deinitialization}2 requires "
       "exclusive access; consider copying to a local variable",
-      (DescriptiveDeclKind, Identifier, unsigned))
+      (DescriptiveDeclKind, StringRef, unsigned))
 
 ERROR(exclusivity_access_required_unknown_decl,none,
         "simultaneous accesses, but "