Disable ASTGen runtime-compat libs

The runtime compat 5.6 library is not available in current toolchains.
The stage-0 compiler builds fine since the builder compiler is not aware
of the 5.6 library, but the stage-1 compiler will not link since the
builder libraries don't have the compat 5.6 library, while the stage-0
compiler is aware of the library and attempts to link it into the built
products. The issue is that the bootstrap goes and uses the libraries
off of the builder instead of the things that actually match the
compiler.

This is currently safe because the 5.6 compat library only contains
concurrency runtime fixes and the compiler frontend does not use
concurrency yet. If this changes, we will need to re-enable this and
make the bootstrap use the libraries correctly.

Author: etcwilde

lib/ASTGen/CMakeLists.txt

@@ -32,6 +32,19 @@ if (SWIFT_SWIFT_PARSER)
     Sources/ASTGen/Types.swift
   )
 
+  # The compat56 library is not available in current toolchains. The stage-0
+  # compiler will build fine since the builder compiler is not aware of the 56
+  # compat library, but the stage-1 and subsequent stage compilers will fail as
+  # the stage-0 compiler is aware and will attempt to include the appropriate
+  # compatibility library. We should turn this back on once we are building the
+  # compiler correctly.
+  # Note: This is safe at the moment because the 5.6 compat library only
+  #       contains concurrency runtime fixes, and the compiler frontend does not
+  #       use concurrency at the moment.
+  target_compile_options(swiftASTGen PRIVATE
+    $<$<COMPILE_LANGUAGE:Swift>:-runtime-compatibility-version>
+    $<$<COMPILE_LANGUAGE:Swift>:none>)
+
   # Set the appropriate target triple.
   if(SWIFT_HOST_VARIANT_SDK IN_LIST SWIFT_DARWIN_PLATFORMS)
     set(DEPLOYMENT_VERSION "${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_DEPLOYMENT_VERSION}")