[SILGen] Map the @_extern(c) C function name over to the asmname of a SIL function

`@_extern(c)` is meant for referencing C functions defined outside of
this Swift file. Instead of using the C function name as the SIL
function name, which is prone to collisions across different Swift
modules, place make the C function name the "asmname" of the
corresponding SIL function. Show that this prevents deserialization
errors when there are conflicting Swift-level types for the same
`@_extern(c)`-named symbol across modules.

Part of rdar://137014448.
Instead of using the C name as the mangled name of a SIL function

test/SIL/Serialization/Inputs/extern_with_nonnull.swift

@@ -0,0 +1,8 @@
+@_extern(c, "takes_a_void_pointer")
+public func takes_a_void_pointer(_ pointer: UnsafeRawPointer)
+
+@_alwaysEmitIntoClient
+public func callWithNonNull() {
+  let pointer = UnsafeMutablePointer<Int>.allocate(capacity: 1)
+  takes_a_void_pointer(UnsafeRawPointer(pointer))
+}

test/SIL/Serialization/Inputs/extern_with_nullable.swift

@@ -0,0 +1,7 @@
+@_extern(c, "takes_a_void_pointer")
+public func takes_a_void_pointer(_ pointer: UnsafeRawPointer?)
+
+@_alwaysEmitIntoClient
+public func callWithNullable() {
+  takes_a_void_pointer(nil)
+}

test/SIL/Serialization/extern_collision.swift

@@ -0,0 +1,18 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -enable-experimental-feature Extern -o %t %S/Inputs/extern_with_nullable.swift
+// RUN: %target-swift-frontend -emit-module -enable-experimental-feature Extern -o %t %S/Inputs/extern_with_nonnull.swift
+// RUN: %target-swift-frontend -emit-sil -o %t -I %t -primary-file %s -module-name main -O
+
+// REQUIRES: swift_feature_Extern
+
+// Don't crash or otherwise fail when inlining multiple functions that reference
+// @_extern(c) declarations of the same name but different types at the SIL
+// level.
+
+import extern_with_nullable
+import extern_with_nonnull
+
+public func main() {
+  callWithNullable()
+  callWithNonNull()
+}