Use RTLD_DEEP_BIND on Linux when opening libsourcekitdInProc

We have multiple copies of llvm symbols (indexstoredb, indexstore,
llbuild via swiftpm, and sourcekitd), so it's important to keep them
isolated. In practice this seems to have started breaking after we
updated swiftpm and llbuild was added, but it was already a potential
issue before that. The most egregious issue is if you build sourcekit
with assertions, it enables ABI-breaking checks in llvm. On macOS this
dlopen behaviour is already the default.

Author: benlangmuir

Sources/SKSupport/dlopen.swift

@@ -47,8 +47,10 @@ public struct DLOpenFlags: RawRepresentable, OptionSet {
   // Platform-specific flags.
   #if os(macOS)
     public static let first: DLOpenFlags = DLOpenFlags(rawValue: RTLD_FIRST)
+    public static let deepBind: DLOpenFlags = DLOpenFlags(rawValue: 0)
   #else
     public static let first: DLOpenFlags = DLOpenFlags(rawValue: 0)
+    public static let deepBind: DLOpenFlags = DLOpenFlags(rawValue: RTLD_DEEPBIND)
   #endif
 
   public var rawValue: Int32

Sources/SourceKit/sourcekitd/SwiftSourceKitFramework.swift

@@ -42,7 +42,7 @@ final class SwiftSourceKitFramework {
 
   init(dylib path: AbsolutePath) throws {
     self.path = path
-    self.dylib = try dlopen(path.asString, mode: [.lazy, .local, .first])
+    self.dylib = try dlopen(path.asString, mode: [.lazy, .local, .first, .deepBind])
 
     func dlsym_required<T>(_ handle: DLHandle, symbol: String) throws -> T {
       guard let sym: T = dlsym(handle, symbol: symbol) else {