Fix MSVC errors compiling GenericSignature.h

[hughbe]

This is because MSVC doesn't support auto function pointers as template parameters and bugs out with the following message:

source_file.cpp(19): error C2275: 'CanType': illegal use of this type as an expression
source_file.cpp(8): note: see declaration of 'CanType'
source_file.cpp(19): error C2146: syntax error: missing ')' before identifier 'dependentType'
source_file.cpp(19): error C2146: syntax error: missing '>' before identifier 'conformedProtocol'
source_file.cpp(22): error C2977: 'function_ref': too many template arguments
source_file.cpp(6): note: see declaration of 'function_ref'

I've also made a connect bug for this: https://connect.microsoft.com/VisualStudio/feedback/details/3116499

[compnerd]

Its not your change, but Im not sure I understand the point of the trailing return with the auto. Also, can you clang-format that line? Pretty sure that it will add a space between -> and the return type.

[hughbe]

This was what clang-format produced for me

[DougGregor]

@swift-ci please smoke test

[compnerd]

@swift-ci please smoke test