AST: Remove ParameterTypeFlags::Escaping

Escapingness is a property of the type of a value, not a property of a function
parameter. Having it as a separate parameter flag just meant one more piece of
state that could get out of sync and cause weird problems.

Instead, always look at the noescape bit in a function type as the canonical
source of truth.

This does mean that '@escaping' is now printed in a few diagnostics where it was
not printed before; we can investigate these as separate issues, but it is
correct to print it there because the function types in question are, in fact,
escaping.

Fixes <https://bugs.swift.org/browse/SR-10256>, <rdar://problem/49522774>.

include/swift/Serialization/ModuleFormat.h

@@ -52,7 +52,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 484; // SDK-relative dependencies flag
+const uint16_t SWIFTMODULE_VERSION_MINOR = 485; // remove @escaping parameter flag
 
 using DeclIDField = BCFixed<31>;
 
@@ -765,7 +765,6 @@ namespace decls_block {
     TypeIDField,        // type
     BCFixed<1>,         // vararg?
     BCFixed<1>,         // autoclosure?
-    BCFixed<1>,         // escaping?
     ValueOwnershipField // inout, shared or owned?
   >;