[RemoteInspection] Read ContextDescriptor from symbol if available

If the MemoryReader can provide a symbol for the context descriptor, use
that instead of building the mangle tree from metadata. This shouldn't
hurt other clients, and would have a few benefits for LLDB:

- Small but numerous memory reads can be expensive for LLDB, building
  the mangle tree from a string should be much faster.

- For types with private discriminators, this allows LLDB to provide the
  same private discriminator that's in DWARF and in the swiftmodule (as
  opposed to using the memory address where the context descriptor lives
  at). This means that these private types would have the same mangled
  name in every run. LLDB persistently caches field descriptors by name,
  but since private discriminators aren't guaranteed to have the same
  mangled name over multiple runs, LLDB would often need to invalidate
  its entire cache and rebuild it. Using the stable UUID would solve
  that issue.

rdar://165950673

Author: augusto2112

include/swift/Remote/MetadataReader.h

@@ -2674,6 +2674,18 @@ class MetadataReader {
       return nullptr;
     }
 
+    // Check if the Reader can provide a symbol for this descriptor, and if it 
+    // can, use that instead.
+    if (auto remoteAbsolutePointer =
+            Reader->resolvePointerAsSymbol(descriptor.getRemoteAddress())) {
+      auto symbol = remoteAbsolutePointer->getSymbol();
+      if (!symbol.empty()) {
+        if (auto demangledSymbol = buildContextManglingForSymbol(symbol, dem)) {
+          return demangledSymbol;
+        }
+      }
+    }
+
     // Read the parent descriptor.
     auto parentDescriptorResult = readParentContextDescriptor(descriptor);