Rollup merge of #77950 - arlosi:sha256, r=eddyb

Add support for SHA256 source file hashing

Adds support for `-Z src-hash-algorithm sha256`, which became available in LLVM 11.

Using an older version of LLVM will cause an error `invalid checksum kind` if the hash algorithm is set to sha256.

r? `@eddyb`
cc #70401 `@est31`

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -558,6 +558,7 @@ pub enum ChecksumKind {
     None,
     MD5,
     SHA1,
+    SHA256,
 }
 
 extern "C" {