RFR: 8301308: Remove version conditionalization for gcc/clang PRAGMA_DIAG_PUSH/POP

[Julian Waters]

As of now we at minimum require clang 3.5 and gcc 6 to compile the Java Platform, the version checks for gcc/clang here are for whether clang is either version 4 and above, or has a minor version higher than 3.1, and for gcc either a major version higher than 4 or minor version above 4.6. Now these will always pass, so they can be removed. Also changes the macro definition location to match Visual C++ and look neater

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Commit messages:
 - Remove version conditionalization for gcc/clang PRAGMA_DIAG_PUSH/POP

Changes: https://git.openjdk.org/jdk/pull/13025/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13025&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301308
  Stats: 13 lines in 1 file changed: 3 ins; 10 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13025.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13025/head:pull/13025

PR: https://git.openjdk.org/jdk/pull/13025