[ClangImporter] Fix enum conversion type when importing global values (unwrap if needed)

[kubamracek]

This fixes a bug in the (recently added) ClangImporter logic that imports global/static integer+enum+float values on top of their declarations. See the attached testcase that triggers a compiler crash:

SIL verification failed: return value type does not match return type of function
  $XCTestError.Code
  $Optional<XCTestError.Code>
Verifying instruction:
     %6 = apply %5(%4, %0) : $@convention(method) (Int, @thin XCTestError.Code.Type) -> Optional<XCTestError.Code> // user: %7
->   return %6 : $Optional<XCTestError.Code>      // id: %7
In function:
// XCTestErrorUnsupported.getter
// Isolation: nonisolated
sil shared [transparent] [serialized] [ossa] @$sSo22XCTestErrorUnsupportedSo0aB4CodeVvg : $@convention(thin) () -> XCTestError.Code {
bb0:
  %0 = metatype $@thin XCTestError.Code.Type      // user: %6
  %1 = integer_literal $Builtin.IntLiteral, 109   // user: %4
  %2 = metatype $@thin Int.Type                   // user: %4
  // function_ref Int.init(_builtinIntegerLiteral:)
  %3 = function_ref @$sSi22_builtinIntegerLiteralSiBI_tcfC : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %4
  %4 = apply %3(%1, %2) : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %6
  // function_ref XCTestErrorCode.init(rawValue:)
  %5 = function_ref @$sSo15XCTestErrorCodeV8rawValueABSgSi_tcfC : $@convention(method) (Int, @thin XCTestError.Code.Type) -> Optional<XCTestError.Code> // user: %6
  %6 = apply %5(%4, %0) : $@convention(method) (Int, @thin XCTestError.Code.Type) -> Optional<XCTestError.Code> // user: %7
  return %6                                       // id: %7
} // end sil function '$sSo22XCTestErrorUnsupportedSo0aB4CodeVvg'

If a C enum is imported as a Swift enum with a failable initializer, we need to unwrap the value after we call the initializer in the getter.

rdar://151174264

[nkcsgexi]

Thank you for the quick fix, Kuba!

[kubamracek]

@swift-ci please test

[j-hui]

Looks good to me!

[beccadax]

Retrospective review: LGTM.