Process: Use more rawStd{Out,Err} const methods instead of mutable ones

Avoid using readAllStandard{Output,Error} mutable methods if possible.
Use non-mutable methods when we are not connected to
readyReadStandard{Output,Error} signals.

Change-Id: I2e830e571b9eab2177fd856bbe06dfc5137d9c01
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/cppeditor/cppcodemodelsettings.cpp

@@ -363,7 +363,7 @@ static FilePath getClangHeadersPathFromClang(const FilePath &clangdFilePath)
     if (!clang.waitForFinished())
         return {};
     const FilePath resourceDir = FilePath::fromUserInput(QString::fromLocal8Bit(
-            clang.readAllRawStandardOutput().trimmed()));
+            clang.rawStdOut().trimmed()));
     if (resourceDir.isEmpty() || !resourceDir.exists())
         return {};
     const FilePath includeDir = resourceDir.pathAppended("include");