clang-format with UseTab: Always sometimes doesn't insert the right amount of tabs.

Trailing comments are not always aligned properly when UseTab is set to Always.

Consider:

int a;        // x
int bbbbbbbb; // x
With .clang-format:

---
Language:        Cpp
BasedOnStyle: LLVM
UseTab: Always
...
The trailing comments of this code block should be aligned, but aren't

To align the first trailing comment it needs to insert 8 spaces. This should be
one tab plus six spaces. It skips the logic of the first partial tab in
FirstTabWidth (=2) + Style.TabWidth (=8) <= Spaces (=8) and only inserts one
tab. Proposed fix and test is attached.

Patch by Hylke Kleve.

Differential revision: https://reviews.llvm.org/D57655

llvm-svn: 354183