ProjectExplorer: Provide a ToolChain::isValid() base implementation

It checks for an executable compilerCommand(). This was used in Gcc,
Clang and Nim before. MSVC still overrides with a different test.

The formerly unconditional 'return true' for Keil/IAR/Sdcc uses that
check now, too.

Change-Id: I433a5ac6784277dc77129b2671c28af6913327a7
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/toolchain.cpp

@@ -159,6 +159,14 @@ Abis ToolChain::supportedAbis() const
     return {targetAbi()};
 }
 
+bool ToolChain::isValid() const
+{
+    if (compilerCommand().isEmpty())
+        return false;
+    QFileInfo fi = compilerCommand().toFileInfo();
+    return fi.isExecutable();
+}
+
 QStringList ToolChain::includedFiles(const QStringList &flags, const QString &directory) const
 {
     Q_UNUSED(flags)