Fix call to templated readFullContextDescriptor method

[j-hui]

This function call was causing an error in my local builds:

In file included from /Users/j-hui/Documents/open2/swift/stdlib/public/RemoteInspection/TypeLowering.cpp:27:
In file included from /Users/j-hui/Documents/open2/swift/include/swift/RemoteInspection/TypeLowering.h:24:
/Users/j-hui/Documents/open2/swift/include/swift/Remote/MetadataReader.h:1524:21: error: use 'template' keyword to treat 'readObj' as a dependent template name
 1524 |       ptr = Reader->readObj<DescriptorTy>(address);
      |                     ^
      |                     template
1 error generated.

Applying the suggested fix seemed to resolve the issue.

[j-hui]

@swift-ci please smoke test