[ABI] Use generic environment to handle mangled generic keypath types.

Always use mangled type names to represent type metadata in keypath patterns.
For generic types, use the generic environment to pull substituted types
from the instantiation arguments.

Finishes the type metadata part of rdar://problem/38038799.

unittests/runtime/Stdlib.cpp

@@ -256,7 +256,9 @@ const long long $ss12_ClassMirrorVMa[1] = {0};
 // KeyPath
 
 SWIFT_RUNTIME_STDLIB_SPI
-const HeapObject *swift_getKeyPathImpl(const void *p, const void *a) {
+const HeapObject *swift_getKeyPathImpl(const void *p,
+                                       const void *e,
+                                       const void *a) {
   abort();
 }