AST/SILGen: Requestify var initializer expression typechecking.

Allow initializer expressions to be emitted during SILGen when
`-experimental-lazy-typecheck` is specified by introducing a new request that
fully typechecks the init expressions of pattern binding declarations
on-demand.

There are still a few rough edges, like missing support for wrapped properties
and incomplete handling of subsumed initializers. Fixing these issues is not an
immediate priority because in the short term `-experimental-lazy-typecheck`
will always be accompanied by `-enable-library-evolution` and
`-experimental-skip-non-exportable-decls`. This means that only the
initializers of properties on `@frozen` types will need to be emitted and
property wrappers are not yet fully supported on properties belonging to
`@frozen` types.

Resolves rdar://117448868

include/swift/AST/Pattern.h

@@ -68,10 +68,11 @@ protected:
   // clang-format off
   union { uint64_t OpaqueBits;
 
-  SWIFT_INLINE_BITFIELD_BASE(Pattern, bitmax(NumPatternKindBits,8)+1+1,
+  SWIFT_INLINE_BITFIELD_BASE(Pattern, bitmax(NumPatternKindBits,8)+1+1+1,
     Kind : bitmax(NumPatternKindBits,8),
     isImplicit : 1,
-    hasInterfaceType : 1
+    hasInterfaceType : 1,
+    executableInitChecked : 1
   );
 
   SWIFT_INLINE_BITFIELD_FULL(TuplePattern, Pattern, 32,
@@ -104,6 +105,7 @@ protected:
     Bits.Pattern.Kind = unsigned(kind);
     Bits.Pattern.isImplicit = false;
     Bits.Pattern.hasInterfaceType = false;
+    Bits.Pattern.executableInitChecked = false;
   }
 
 private:
@@ -136,6 +138,11 @@ public:
   bool isImplicit() const { return Bits.Pattern.isImplicit; }
   void setImplicit() { Bits.Pattern.isImplicit = true; }
 
+  bool executableInitChecked() const {
+    return Bits.Pattern.executableInitChecked;
+  }
+  void setExecutableInitChecked() { Bits.Pattern.executableInitChecked = true; }
+
   /// Find the smallest subpattern which obeys the property that matching it is
   /// equivalent to matching this pattern.
   ///