[clang] Remove misleading assertion in FullSourceLoc

D31709 added an assertion was added to `FullSourceLoc::hasManager()` that ensured a valid `SourceLocation` is always paired with a `SourceManager`, and missing `SourceManager` is always paired with an invalid `SourceLocation`.

This appears to be incorrect, since clients never cared about constructing `FullSourceLoc` to uphold that invariant, or always checking `isValid()` before calling `hasManager()`.

The assertion started failing when serializing diagnostics pointing into an explicit module. Explicit modules don't have valid `SourceLocation` for the `import` statement, since they are "imported" from the command-line argument `-fmodule-name=x.pcm`.

This patch removes the assertion, since `FullSourceLoc` was never intended to uphold any kind of invariants between the validity of `SourceLocation` and presence of `SourceManager`.

Reviewed By: arphaman

Differential Revision: https://reviews.llvm.org/D106862