Manually mark 1.49 and 1.50 in linker-plugin-lto.md as Clang 11

`rustc +1.49.0 -Vv` and `rustc +1.50.0 -Vv` do not print out an
`LLVM version` line, which prevents the script from detecting them.

Author: str4d

src/doc/rustc/src/linker-plugin-lto.md

@@ -181,8 +181,8 @@ The following table shows known good combinations of toolchain versions.
 | Rust 1.46    |    Clang 10   |
 | Rust 1.47    |    Clang 11   |
 | Rust 1.48    |    Clang 11   |
-| Rust 1.49    |    Clang      |
-| Rust 1.50    |    Clang      |
+| Rust 1.49    |    Clang 11   |
+| Rust 1.50    |    Clang 11   |
 | Rust 1.51    |    Clang 11   |
 | Rust 1.52    |    Clang 12   |
 | Rust 1.53    |    Clang 12   |