Switch from clang::ASTUnit to clang::CompilerInstance. (#6483)

This gives us a lot more control over how the compiler is built and
invoked. But no functionality changes are intended in this PR.

toolchain/check/cpp/context.cpp

@@ -8,7 +8,8 @@
 
 namespace Carbon::Check {
 
-CppContext::CppContext(clang::ASTUnit* ast_unit) : ast_unit_(ast_unit) {}
+CppContext::CppContext(std::unique_ptr<clang::FrontendAction> action)
+    : action_(std::move(action)) {}
 
 CppContext::~CppContext() = default;