[clang][frontend] Require invocation to construct `CompilerInstance` (#137668)

This PR makes it so that `CompilerInvocation` needs to be provided to
`CompilerInstance` on construction. There are a couple of benefits in my
view:
* Making it impossible to mis-use some `CompilerInstance` APIs. For
example there are cases, where `createDiagnostics()` was called before
`setInvocation()`, causing the `DiagnosticEngine` to use the
default-constructed `DiagnosticOptions` instead of the intended ones.
* This shrinks `CompilerInstance`'s state space.
* This makes it possible to access **the** invocation in
`CompilerInstance`'s constructor (to be used in a follow-up).

1 files changed, 1 insertions, 2 deletions

diff --git a/clang/unittests/Frontend/CompilerInstanceTest.cpp b/clang/unittests/Frontend/CompilerInstanceTest.cpp
index 07329eb2..6e9a6f5 100644
--- a/clang/unittests/Frontend/CompilerInstanceTest.cpp
+++ b/clang/unittests/Frontend/CompilerInstanceTest.cpp
@@ -66,9 +66,8 @@ TEST(CompilerInstance, DefaultVFSOverlayFromInvocation) {
     FAIL() << "could not create compiler invocation";
   // Create a minimal CompilerInstance which should use the VFS we specified
   // in the CompilerInvocation (as we don't explicitly set our own).
-  CompilerInstance Instance;
+  CompilerInstance Instance(std::move(CInvok));
   Instance.setDiagnostics(Diags.get());
-  Instance.setInvocation(CInvok);
   Instance.createFileManager();
 
   // Check if the virtual file exists which means that our VFS is used by the