Add an error message when trying to remove valued enum attributes

The current version of LLVM crashes when attempting to remove valued
enum attributes. The bug was discovered and fixed in LLVM 7 (see
llvm-mirror/llvm@9bc0b10). The guard placed
in `removeAttribute` should be removed once this version is released.

Author: kyouko-taiga

Sources/LLVM/Function+Attributes.swift

@@ -343,6 +343,17 @@ extension Function {
   /// - parameter index: The index representing the function, its return value
   ///   or one of its parameters.
   public func removeAttribute(_ attrKind: AttributeKind, from index: AttributeIndex) {
+    // FIXME: Remove when LLVM 7.0.0 is released.
+    guard  attrKind != .align
+        && attrKind != .alignstack
+        && attrKind != .allocsize
+        && attrKind != .dereferenceable
+        && attrKind != .dereferenceableOrNull else {
+          fatalError(
+            "Removing valued enum attributes crashes in LLVM <7.0.0 " +
+            "and is currently disabled in LLVMSwift.")
+    }
+
     LLVMRemoveEnumAttributeAtIndex(llvm, index.rawValue, attrKind.id)
   }
 

Tests/LLVMTests/IRAttributesSpec.swift

@@ -131,10 +131,7 @@ class IRAttributesSpec : XCTestCase {
       XCTAssertEqual(enumAttr.value, value)
       XCTAssert(fn.attributes(at: .function).contains { $0.asLLVM() == enumAttr.asLLVM() })
 
-      // LLVM issue?
-      // Removing an integer attribute seems to always fail, because
-      // `AttributeList::removeAttribute` (Attributes.cpp:1096) attempts to add the
-      // attribute about to be removed without preserving its value.
+      // FIXME: Remove when LLVM 7.0.0 is released.
       guard value == 0 else { continue }
 
       fn.removeAttribute(enumAttr, from: .function)
@@ -198,7 +195,7 @@ class IRAttributesSpec : XCTestCase {
       XCTAssertEqual(enumAttr.value, value)
       XCTAssert(fn.attributes(at: .returnValue).contains { $0.asLLVM() == enumAttr.asLLVM() })
 
-      // LLVM issue?
+      // FIXME: Remove when LLVM 7.0.0 is released.
       guard value == 0 else { continue }
 
       fn.removeAttribute(enumAttr, from: .returnValue)
@@ -214,7 +211,7 @@ class IRAttributesSpec : XCTestCase {
       XCTAssertEqual(enumAttr.value, value)
       XCTAssert(fn.attributes(at: .argument(0)).contains { $0.asLLVM() == enumAttr.asLLVM() })
 
-      // LLVM issue?
+      // FIXME: Remove when LLVM 7.0.0 is released.
       guard value == 0 else { continue }
 
       fn.removeAttribute(enumAttr, from: .argument(0))