[5.5] [SymbolGraph][Driver] add symbol-graph flag to include SPI symbols

[QuietMisdreavus]

Resolves rdar://70794131

This PR ports #37640 to release/5.5.

Change Information

• Explanation: Adds a new field to the symbol graph, spi, and a new CLI flag, -include-spi-symbols, which facilitate generating symbol graphs with SPI symbols.
• Scope of Issue: Currently, the only way to include SPI symbols in the symbol graph is to set the "minimum access level" to "internal" or higher, which adds a lot of excess symbols to the graph. In addition, there is currently no way to determine whether a given symbol is SPI or not.
• Risk: Low.
• Automated Testing: A new lit test, "SymbolGraph/Symbols/Mixins/SPI", has been added. PR testing has passed.

[QuietMisdreavus]

On a whim, i'm going to try to test this, since i can get the test's command to work locally (the swiftc call that adds -include-spi-symbols) in the Xcode build, but not the Ninja build. If it works in CI, i'll call this ready to review.

@swift-ci Please test

[QuietMisdreavus]

Turns out my testing issue was a local configuration problem. After i did a clean rebuild of the compiler, everything seems to be working on my system. Marking this as ready.

[QuietMisdreavus]

@swift-ci Please build toolchain macOS platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - fd26b86

[swift-ci]

macOS Toolchain
Download Toolchain
Git Sha - fd26b86

Install command
tar -zxf swift-PR-37641-1002-osx.tar.gz --directory ~/

[QuietMisdreavus]

The failing test in the macOS CI is flaky and is being disabled in swiftlang/llvm-project#3005.

@swift-ci Please test macOS platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - fd26b86

[QuietMisdreavus]

It looks like that PR was merged while the CI was running, which caused an error in cloning LLVM.

@swift-ci Please test macOS platform