Fix SILInliner Compilation on MSVC

[gmittert]

SILBuilder contains a SILBuilderContext which contains a SILModule which
has a deleted operator=. Assigning an Optional<BeginApplySite> causes MSVC
to attempt to create operator= for SILBuilder which fails due to the
deleted operator=. This changes BeginApplySite to use a SILBuilder*
instead of a SILBuilder& to get around it.

Previously:

C:\Users\jmittertreiner\LocalSwift\llvm\include\llvm/ADT/Optional.h(54): error C2280: 'swift::BeginApplySite &swift::BeginApplySite::operator =(const swift::BeginApplySite &)': attempting to reference a deleted function
C:\Users\jmittertreiner\LocalSwift\swift\lib\SILOptimizer\Utils\SILInliner.cpp(226): note: compiler has generated 'swift::BeginApplySite::operator =' here
C:\Users\jmittertreiner\LocalSwift\swift\lib\SILOptimizer\Utils\SILInliner.cpp(226): note: 'swift::BeginApplySite &swift::BeginApplySite::operator =(const swift::BeginApplySite &)': function was implicitly deleted because 'swift::BeginApplySite' has a data member 'swift::BeginApplySite::Builder' of reference type
C:\Users\jmittertreiner\LocalSwift\swift\lib\SILOptimizer\Utils\SILInliner.cpp(75): note: see declaration of 'swift::BeginApplySite::Builder'
C:\Users\jmittertreiner\LocalSwift\llvm\include\llvm/ADT/Optional.h(52): note: while compiling class template member function 'llvm::optional_detail::OptionalStorage<T,false> &llvm::optional_detail::OptionalStorage<T,false>::operator =(T &&)'
        with
        [
            T=swift::BeginApplySite
        ]
C:\Users\jmittertreiner\LocalSwift\llvm\include\llvm/ADT/Optional.h(65): note: see reference to function template instantiation 'llvm::optional_detail::OptionalStorage<T,false> &llvm::optional_detail::OptionalStorage<T,false>::operator =(T &&)' being compiled
        with
        [
            T=swift::BeginApplySite
        ]
C:\Users\jmittertreiner\LocalSwift\llvm\include\llvm/ADT/Optional.h(132): note: see reference to class template instantiation 'llvm::optional_detail::OptionalStorage<T,false>' being compiled
        with
        [
            T=swift::BeginApplySite
        ]
C:\Users\jmittertreiner\LocalSwift\swift\lib\SILOptimizer\Utils\SILInliner.cpp(93): note: see reference to class template instantiation 'llvm::Optional<swift::BeginApplySite>' being compiled

[gottesmm]

Just a meta comment (this is not a functionality review). Can you run this through git-clang-format? Swift c++'s style uses Type *, not Type*

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 64df7671ffaa4c884e6c392172980de8b57bc1dc

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 64df7671ffaa4c884e6c392172980de8b57bc1dc

[compnerd]

@swift-ci please test macOS platform

[atrick]

LGTM