Fix cache collision between object file and precompiled headers

When compiling with PCH enabled, it happens that some times object files
end up with PCH content, or that .gch files end up with object code.

Simplest way to reproduce the problem:

  $ touch empty.c
  $ gcc              -c -o out1 empty.c
  $ gcc  -x c-header -c -o out2 empty.c
  $ file out1 out2
  out1: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
  out2: GCC precompiled header (version 014) for C

The two files are different. But if we feed these compilations to
sccache, they lead to the same result:

  $ sccache gcc              -c -o out3 empty.c
  $ sccache gcc  -x c-header -c -o out4 empty.c
  $ file out3 out4
  out3: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
  out4: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

Same thing is reproducible with c++ compiler and -x c++-header argument.

The reason is that the hash string that identifies each command line is
the same.

With this patch, compilation of C files is always differentiated from
compilations of same C-Header files. And compilation of C++ files is
always differentiated from compilations of same C++-Header files.

Fixes #1851.

src/compiler/c.rs

@@ -1448,6 +1448,26 @@ mod test {
         );
     }
 
+    #[test]
+    fn test_header_differs() {
+        let args = ovec!["a", "b", "c"];
+        const PREPROCESSED: &[u8] = b"hello world";
+        assert_neq!(
+            hash_key("abcd", Language::C,       &args, &[], &[], PREPROCESSED, false),
+            hash_key("abcd", Language::CHeader, &args, &[], &[], PREPROCESSED, false)
+        );
+    }
+
+    #[test]
+    fn test_plusplus_header_differs() {
+        let args = ovec!["a", "b", "c"];
+        const PREPROCESSED: &[u8] = b"hello world";
+        assert_neq!(
+            hash_key("abcd", Language::Cxx,       &args, &[], &[], PREPROCESSED, true),
+            hash_key("abcd", Language::CxxHeader, &args, &[], &[], PREPROCESSED, true)
+        );
+    }
+
     #[test]
     fn test_hash_key_executable_contents_differs() {
         let args = ovec!["a", "b", "c"];

src/compiler/compiler.rs

@@ -146,8 +146,10 @@ impl Language {
 
     pub fn as_str(self) -> &'static str {
         match self {
-            Language::C | Language::CHeader => "c",
-            Language::Cxx | Language::CxxHeader => "c++",
+            Language::C => "c",
+            Language::CHeader => "cHeader",
+            Language::Cxx => "c++",
+            Language::CxxHeader => "c++Header",
             Language::GenericHeader => "c/c++",
             Language::ObjectiveC => "objc",
             Language::ObjectiveCxx => "objc++",