Add missing <string_view> and <unordered_set> includes.

comgr-compiler.cpp declares

    static inline const std::unordered_set<std::string_view> ValidSpirvFlags{...}

at file scope but includes neither header, relying on them arriving
transitively through an LLVM header. That does not hold here and the build
fails with:

    error: 'unordered_set' in namespace 'std' does not name a template type
    error: 'ValidSpirvFlags' was not declared in this scope

Note this declaration is NOT inside the COMGR_SPIRV_TRANSLATOR_AVAILABLE
guard, so it is compiled even when configuring with -DCOMGR_DISABLE_SPIRV=ON
as this ebuild does.
--- a/src/comgr-compiler.cpp
+++ b/src/comgr-compiler.cpp
@@ -78,6 +78,8 @@
 
 #include <csignal>
 #include <sstream>
+#include <string_view>
+#include <unordered_set>
 
 LLD_HAS_DRIVER(elf)