ClangCodeModel: Fix header file completion weirdness

... when completing right after a dot.
It's unclear why the code uses half-baked heuristics all over the place
instead of simply employing the lexer (which is also used at some point)
for everything. For now, use it to determine the correct starting
location of string literals.

Fixes: QTCREATORBUG-25154
Change-Id: I14c4d71c4a9bc3e2407688ed9c908d1c99155c76
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangactivationsequencecontextprocessor.cpp

@@ -282,7 +282,22 @@ int ActivationSequenceContextProcessor::findStartOfName(
 
 void ActivationSequenceContextProcessor::goBackToStartOfName()
 {
-    m_startOfNamePosition = findStartOfName(m_assistInterface, m_positionInDocument);
+    CPlusPlus::SimpleLexer tokenize;
+    tokenize.setLanguageFeatures(m_assistInterface->languageFeatures());
+    tokenize.setSkipComments(false);
+    const int state = CPlusPlus::BackwardsScanner::previousBlockState(m_textCursor.block());
+    const CPlusPlus::Tokens tokens = tokenize(m_textCursor.block().text(), state);
+    const int tokenPos = std::max(0, m_textCursor.positionInBlock() - 1);
+    const int tokIndex = CPlusPlus::SimpleLexer::tokenAt(tokens, tokenPos);
+    if (tokIndex > -1 && tokens.at(tokIndex).isStringLiteral()) {
+        const int tokenStart = tokens.at(tokIndex).utf16charOffset;
+        const int slashIndex = m_textCursor.block().text().lastIndexOf('/',
+            std::min(m_textCursor.positionInBlock(), m_textCursor.block().text().length() - 1));
+        m_startOfNamePosition = m_textCursor.block().position() + std::max(slashIndex, tokenStart)
+                + 1;
+    } else {
+        m_startOfNamePosition = findStartOfName(m_assistInterface, m_positionInDocument);
+    }
 
     if (m_startOfNamePosition != m_positionInDocument)
         m_textCursor.setPosition(m_startOfNamePosition);